Key thing to remember here... the OS that ships with the system anew is the OS that was written for said system. Just because it is capable of running later OS versions doesn't mean that you should. Everyone's use case is different.
For the record, Windows has been traditionally backwards compatible... which is why it is so bloated. So while you may be running a newer version of the OS, it is often emulating an older version on the fly... which also tends to slow it down. Mac OSes tend to throw out support for older hardware like bath water... by design.
Apple is in the business of selling you new hardware... which is why the OS is free. You upgrade your system to the latest OS and it starts to show the age of your system. Encouragement to buy a new computer... the cycle repeats itself for all perpetuity.
Keep in mind, official support for 32 bit applications ends at Mojave.
Apple isn't going to fix issues with their older OSes unless held at gunpoint. At this point in time, unless you are considering an ARM based Mac, you are no longer on their radar.
I ran my 2010 Mac Pro tower up until November of last year when I decided to return to the Windows world instead of buying an ARM Mac. For me, it was the right decision. I was a Mac user for well over 2 decades and never had any regrets about it. Like you, I preferred Mojave over the later incarnations of the OS. Since it was apparent that the direction of the OS and architecture was diverging from my ideal setup, I decided to go where the solutions suited me best.
Define your idea of a desktop computer and what you intend to use it for. Sounds like you are mainly a laptop person. You want a full keyboard and a mouse and larger display at your fingertips? What is it that your laptop can't do that a desktop can? Remember with a dock, you can basically use a laptop like a desktop, you just need to have all the peripherals to do so.
Your best deals are likely to be on eBay no matter how you look at it. Any used Mac sold from a retailer is going to be overpriced. Basically you are working with a sold as-is marketplace no matter what.
I don't want to go back to windows, even Big Sur isn't as bad as Windows 10 I still have my Windows 7 64bit desktop and it is or was a top the line Gaming Machine but since support for windows 7 is completely going away very shortly it's already out dated. I could probably run Windows 10 on it, as it want's me to upgrade but I don't want Windows 10. Maybe Windows 11 is better you know like 7-11 maybe those two have somethings in common IDK.
Then I don't know about on a ARM Mac but on a Intel Mac you can still run 32 bit applications in Big Sur as long as they are Windows Apps using Porting Kit, but there are several 32 bit only Mac Apps that I prefer to use some that were made just for Snow Leopard like Boxer that won't run on anything past Mojave.
I guess the one thing about a Silicon Mac is the fact that it can install and run iOS apps "That would be a nice feature to test out", but with Big Sur the Music App that they replaced iTunes with is the most annoying App you'll ever encounter in your life it'll keep opening up all the time even though you did nothing to activate it and it never goes away even if you never used it, until you employee a hack to stop it from running. Other wise there is no known official fix for it, and Apple hasn't fixed it AFAIK. I don't think they care about it annoying you.
However with these newer MacBook's all having builtin battery's it makes a Desktop Mac seem more Appealing. I have also been considering the prospect of getting an Amiga, I keep seeing people swearing by them online and they seem like very nice Machines to have and the Graphic's of some Amiga games even look better than that same game on an Arcade Machine.
I think I would prefer to have a Mac though I learn new tricks I can do on Mac OS everyday and it's not like Windows were you'll just end up crashing at some point. When your Mac does crash it comes back with everything intact.
You sure I can't win you over to Apple Silicon? It's really hard to justify being on anything less than Big Sur nowadays with how much the ARM Macs kick ass.
I wouldn't worry too much, one these days I'll probably get an ARM Mac with Ventura, for now I just am staying away from Big Sur or later until they finish with updating them. I am far from giving up on Apple and although I still use some 32 bit Apps, even with Windows a lot stuff is coming out as 64 bit only, so 64 bit is the Future anyway.
If I could ever figure out how you can install and run 64 bit Windows Apps on Mac OS I would have it made.