Monterey will get security updates for two more years. Generally speaking you get support for 7 years after the product is discontinued. But even then you can continue to use it until it no longer meets your needs. I still use a 2009 and 2 2012 Mac minis.
That said, I do feel your future pain and hope they do come out with a new 27" iMac. There is a real hole between the old $1900 iMac starting price and the $4000 of a Mac Studio, Studio Display, keyboard and mouse, and that’s where you have to go if you want more than 16GB of RAM. I bought the last generation 27" iMac because I also wanted an Intel processor to run Windows and Linux virtual machines. You can probably still find one on the Refurbished store if you want a few more years of support.
As to your questions.
-
ANY current Mac will outperform your iMac. My wife’s 24" M1 iMac, the low end of the M1 line, outperforms my 2020 27" i7 iMac. No charts or graphs, just side by side observation.
-
More cores only matter for programs and applications that can take advantage of them by dividing the work among them. 10 cores would be at the most 25% faster than 8 cores, but for most use no faster. Photoshop will use them, but how heavy is your processing? The most noticeable improvement would be if you were doing videos.
-
Look in Activity Monitor with your heaviest workload and see how much of that memory you are using. That will tell you if you need more than what you can get in a mini. Of course if an app is running but not actively being used it can be safely swapped out, so the figure is somewhat pessimistic. I’m using 20GB so might get by with 16GB, but would sure be happier with the (unavailable) 32GB.
-
It looks like an iPhone with iOS 16 and Ventura on the Mac will give a better video camera than the Studio Display which appears to have issues.