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Mine has an SSD too (with 16GB or RAM) and Big Sur was possibly the smoothest OS I've ever run on this machine, certainly better than Mojave or Catalina. Not sure how it would work with an HDD through.
I dont use anything platter based anymore other than the drives connected to my X1X and PS4P. All my Macs and PCs are SSD now. But yeah 11.x ran amazing on my 2012 lineup MBP/Air/Mini.
 
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I dont use anything platter based anymore other than the drives connected to my X1X and PS4P. All my Macs and PCs are SSD now. But yeah 11.x ran amazing on my 2012 lineup MBP/Air/Mini.
Same, jumped to SSDs during the past decade and never looked back. Since then, all my devices are SSD based (iPad Pro, 2014 Mac mini, iPhone…)

The only plate drives I still own are external storage and even those I’m slowly replacing them for T5 Samsung SSD drives
 
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Same, jumped to SSDs during the past decade and never looked back. Since then, all my devices are SSD based (iPad Pro, 2014 Mac mini, iPhone…)

The only plate drives I still own are external storage and even those I’m slowly replacing them for T5 Samsung SSD drives
I still have a few external HDDs where SSD would be too expensive (I have drives totalling 16TB for my personal photos). Internally though I've been solely on SSDs for about a decade now. Even my 2003 PowerBook G4 has a 512GB mSATA SSD in an IDE enclosure. An SSD is the best upgrade one can make on any machine that doesn't have one. Installing one on a 2012 mini is a bit involved but not too difficult.
 
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I still have a few external HDDs where SSD would be too expensive (I have drives totalling 16TB for my personal photos). Internally though I've been solely on SSDs for about a decade now. Even my 2003 PowerBook G4 has a 512GB mSATA SSD in an IDE enclosure. An SSD is the best upgrade one can make on any machine that doesn't have one. Installing one on a 2012 mini is a bit involved but not too difficult.
I have an mSata in all 3 of my powerbooks, best upgrade ever.
 
Today I got my Mac Mini, and it's currently running on 16GB of RAM, has a 500HDD, and is on Mountain Lion (10.8.5.). It's a flying machine, and I am happy with my purchase so far. I would keep in on ML if I could, but not everything is supported. In terms of compatibility and performance, which would be the best operating system to put on it? I don't want to lose performance, but I still want compatibility.
Always use the latest one that it'll run - Catalina in this case. Just be aware that one is now unsupported by Apple, and has known security vulnerabilities. I wouldn't do anything critical (i.e. shop, email, other private activities) on it.
 
Not sure how you define "expensive". A quick look showed new 500gb Samsung T7 USB SSD's for $70 at Best Buy. Shop around, I'm sure there are even less expensive options, although I have quite a few Samsung T7's and use them heavily with no issues. Just be sure to reformat them for MacOS (APFS if using Mojave or Catalina).

Oh yeah, I had a look at them, and I could give those a go. They're within my budget, but I thought that they would be more expensive than that, based on what I looked up. External SSD are something I have never experimented with.
 
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My absolute favourite Mac of all time! I have three of them and they still work surprisingly well for so much.

+1 for running Ventura (or Monterey!) off of an external USB 3 SSD. Just grab a cheap SSD & a cheap enclosure. No need to get fancy or even open it up. Even an external SSD will be way faster than an HDD! 512GB+ if possible, but one of mine has 256GB & it's plenty for my needs. And you can always use the internal HDD for storage!

And I was genuinely shocked at how well Ventura runs on my i7/16GB/500GB SSD 2012s... OpenCore Legacy Patcher is amazing! (I do still have a soft spot for Catalina & High Sierra, though.)
 
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Be warned: I had a mid-2010 MacBook Pro that was rendered just shy of unusable after an OS update -- it was either 10.11 or 10.12. Sadly, I cannot recall which of the two it was. A RAM upgrade to 8GB and an SSD upgrade from the HDD brought it back to life, though it's not what I'd call snappy. But it still served me well as my daily for over five years after that point.

Your computer is a little newer than mine. I'm just letting you know -- don't expect to upgrade and have the exact same performance as you have on Mountain Lion. It's unlikely that will be the case.
 
I would say Mojave is the best in terms of compatibility (at least for 2 years)

High Sierra is currently the minimum OS for most software, so going to Mojave would give you at least a little bit of life for 2 years.

Catalina is not one I would update to on that Mac since Apple started paving the way for Apple Silicon on Catalina, (eg. not allowing 32-bit apps to run so Rosetta is a lot better on the Apple Silicon Macs).

Mojave has also introduced a lot of features that are vital for continuity for me, like using your iPhone/iPad with a lot of software, Dark Mode, Stacks, and an easier-to-use Finder.

Even though you probably don't use them, you also have the ability to run 32 Bit apps on Mojave.

Catalina's only benefit is that it has Apple Music and Sidecar, but your 2019 Mac is on Mojave, so you probably haven't even used Sidecar yet.

Also, I would recommend upgrading to an SSD if you were to go to an at least usable OS.
 
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High Sierra is the way to go. Especially with SSD. I run it on many of my 2010 era Macs. Software is basically all compatible, though recently some have started requiring 10.15 Catalina. But screw them.
 
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Be warned: I had a mid-2010 MacBook Pro that was rendered just shy of unusable after an OS update -- it was either 10.11 or 10.12. Sadly, I cannot recall which of the two it was. A RAM upgrade to 8GB and an SSD upgrade from the HDD brought it back to life, though it's not what I'd call snappy. But it still served me well as my daily for over five years after that point.

Your computer is a little newer than mine. I'm just letting you know -- don't expect to upgrade and have the exact same performance as you have on Mountain Lion. It's unlikely that will be the case.
The jump in performance between the Core 2 Duo and Core i5/i7 chips from 2010 to 2011 and onwards was pretty substantial. I had a personal 2009 white MacBook (2.13GHz C2D) at the time when I received a 2011 MacBook Pro (2.3GHz i5) from work, the difference between the two in CPU performance was huge. The 2012 machines still run well today on modern OSs, at least for the time being. Part of this is due to the Intel chips stagnating for several years in terms of performance. My 2020 MacBook Pro with a 10th gen i5 wasn’t much faster than my 2013 MBP with a 4th gen i5. The M1 on the other hand blew them both out of the water.
 
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Literally, not even 2 weeks ago, I retired my 2012 Mac mini (for a new 2023 M2 one) - and that was only because it was overheating (new fan/paste didn't fix...).

DEFINITELY use Mojave on yours. And 1000% get a SSD if it doesn't have one (they're literally like $25 now for a 256/240gb one - which is all I ever needed internally). SSD is a game changer in an older machine like this.
 
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The jump in performance between the Core 2 Duo and Core i5/i7 chips from 2010 to 2011 and onwards was pretty substantial. I had a personal 2009 white MacBook (2.13GHz C2D) at the time when I received a 2011 MacBook Pro (2.3GHz i5) from work, the difference between the two in CPU performance was huge. The 2012 machines still run well today on modern OSs, at least for the time being. Part of this is due to the Intel chips stagnating for several years in terms of performance. My 2020 MacBook Pro with a 10th gen i5 wasn’t much faster than my 2013 MBP with a 4th gen i5. The M1 on the other hand blew them both out of the water.
No doubt regarding the jump in performance from C2D to Core i5/i7 CPU’s. But what I found notable was the fact that the SSD and RAM was what saved my computer. The CPU stayed the same. That’s why I was trying to caution the OP.
 
Ivy Bridge Macs (with SSDs) can run Monterey absolutely fine. Though you need to go the OCLP route for it to work.

I would spend the effort considering it's nice to have both SSD and an operating system that receives security patches.

Later down the line it can probably run Ventura. I'd summarise the Ventura support on Ivy Bridge through OCLP as not completely without its issues right now, though probably at some point in time those things will probably get sorted.
 
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I also vote for replacing the HDD with an SSD. Before Christmas OWC had a 480 GB one for sale for $40.

My 2012 had a fusion drive which worked pretty well with Catalina. After disemboweling it to replace the HDD I reloaded the SSD with Mohave just so I could run an old 32 bit app if need be, and then I loaded Linux Mint XFCE on the 128 GB SSD portion of the former fusion drive.

Both OS's work really well.

The HDD is in an external USB 3 case for backup purposes.
 
Best OS is whatever works best for your workflow—my 2009 Mac Pro and 2014 MacBook Pro are both on Mojave because of the stability, the performance, and the compatibility for the applications I use.

I don’t know when the limitations will hit Mojave, but if you use browsers like Brave, Edge, Firefox, Chrome, Orion (vote on Orion because it’s super minimal, no telemetry, runs on WebKit and does Chrome extensions) you’ll still be getting an updated browser that is still getting security updates. Most of these browsers have a minimum version of 10.13 (High Sierra). Just be smart about where you’re browsing, downloading files, etc. and you can still use an “unsupported” version of Mac OS.
 
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I upgraded from a 2012 i5 to an m2

Put an ssd in there , the biggest you can afford and run the latest macOS which is Catalina, runs like butter
 
Same, jumped to SSDs during the past decade and never looked back. Since then, all my devices are SSD based (iPad Pro, 2014 Mac mini, iPhone…)

The only plate drives I still own are external storage and even those I’m slowly replacing them for T5 Samsung SSD drives
I put an ssd in everything I have including ps4 ps3
 
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