Thanks all. In the end, I went with a previous model 27" iMac, available unopened at a very good price…
Now I have to find a way to mount it below the desktop or else shorten the legs of the computer desk. Otherwise, looking at the top of the screen is going to put me in traction somewhere!
If you lower the legs on your desk the keyboard is also lowered. I’m guessing you may then have better arm/hand position to avoid RSI and body twisting issues.
Raising your chair can do the same if dangling legs are not a problem.