Same dilemma here, I also have a late 2015 5k 27" imac. I’m always I year behind upgrading OS so I’m only going into Monterey in the next month. It should be another two years before apps stop support for Monterey and then I’ll re-look at what I need.
With the pace of technology change I’ve come to the conclusion of replacing more often and buying base models. In terms of screen there are plenty of 5k screens out there, I even know some people who use a 5k tv as you get a larger screen for your money; which might solve your 27" problem. You might want to consider two smaller screens, one in portrait for less scrolling.
This discussion may help your decision although it’s focused on a macbook:
One thing I don’t know: can another mac use my imac 27" screen for display?