The 27" studio monitor is nice although expensive - if I purchase it, then do I get a Mac Mini, or the Studio?
If the 27" iMac were still available, which version would you buy?
If you'd have got the highest end, $3200 model and upgraded the RAM to 32GB at Apple prices then you'd have been pretty close the price of a M1 Max Studio + Studio Display (OK, so I'd have got a cheaper third party RAM upgrade, but then I'd probably
not get the studio display). So, Mac Studio.
If you'd been sniffing around the iMac Pro with the higher CPU options (the base model isn't much faster than the top-end iMac) then it's probably the M1 Ultra Studio.
If the $1800 i5 version would have done, then a Mac Mini would probably be a decent replacement (or a 24" M1 iMac).
If you'd have gone for one of the intermediate models in the $2000-$3000 range then that's a dilemma.
My inclination would be to get the Studio if you have the money.
What's really annoying is that we have no idea if a M2 Mini is coming this year, next year, sometime or never - but the M1 Mini is now 16 months old, so buying one right now wouldn't be my Plan A.
In either case, that leaves the issue of the display.
If I was playing $1600 for a display I'd want it to be a 5-10 year investment. Problem is the edge-lit LCD tech in the Studio Display sounds like it is little different to the display in the old 5k iMac. Now, there is absolutely nothing wrong with the current 5k iMac display
today but it is 5+ year-old tech that could look old very rapidly if mini-LED, micro-LED or faster frame rates come along.
Personally, I'd look around for 3rd party alternatives - and look for
change like ultra-wide, 3:2, multiple smaller displays,
larger 4k displays etc. because a 27" 4k display is always going to feel like a
slight downgrade from 5k. However, remember that 4k@17" is still "retina" at desktop distances, so you shouldn't see obvious pixels, "looks like 2560x1440" is actually 5k downsampled to 4k and is really very good, the GPU load of Scaled mode shouldn't bother a M1 Max, and that once you go above about 28" "1:1" mode in 4k starts to get usable.
If you really must have a display that matches your old iMac, though, the Studio Display is the only game in town (which is why Apple can get away with askiung $1600 for it).
I think the best option is a 14" MBP M1 Pro (which I have) with a Studio display.
If you have a strong use case for having a laptop, getting a MacBook and an external display is a perfectly reasonable plan idea.
If not, with the 14" you're paying the same price as the M1 Max Studio for a binned 8-core-CPU, 14-core-GPU Pro machine, with half the RAM (half the RAM bandwidth), and fewer I/O ports. Also be aware that
if a M2 Mini appears in the next few months, it's likely to have dangerously close, maybe better, performance to the 8/14 core M1 Pro at a cheaper price. (I doubt it will be faster than the 10/16 core Pro but the cheaper, binned Pros could be in trouble)