I updated my 5 year old imac to Monterrey and it rendered it almost unusable, everything is slow as molassas. Slower actually. What used to take 10 minutes now takes 2 hours. It seems to be incompatible with the older machine.
How infuriating.
And will no doubt take hours to troubleshoot, which is also infuriating.
But ... don't give up yet?
I recently upgraded an even older MacBook, and it went surprisingly smoothly. (FWIW, I went straight from Catalina to Monterrey.)
If the following seems more frustrating than useful, just ignore it -- this sort of kibbitzing can be a pita, I know -- but here are a few random ideas in case they help.
(1)
Are you sure it's really done installing? I seem to recall that I kept thinking it was done, but then it would do more upgrade-related stuff. The process messages (especially "time: about 1 minute" HAHAHA) often don't seem accurate. My upgrade may have gone smoothly, but it did not go quickly.
(2)
Could there be a specific program that's hanging it up? Have you tried looking at the Activity Monitor (under Go > Utilities) to see if something weird is going on?
(3)
I don't know whether you use Adobe products, but FWIW I finally did a clean-out of alll my Creative Suite program files (which have been unusable since Catalina anyway). I suppose there's a .00000001% chance that some obsolete program could cause a hang.
(4)
Have you tried rebooting in Safe Mode?
(5)
If worse comes to worst, there are ways to revert back to an older OS.
I do think it's worth venturing into Google/Forum Time-Suck Hell to see if you can find a solution, or at least see what other people have been experiencing who have a system similar to yours. Because my machine is so much (two years) older than yours, and because if it's truly incompatible Monterrey "should" just tell you so and refuse to download, I am guessing there's going to be some kind of a solution.
Meanwhile, try not to grind your teeth. Good luck!