A_Sunbeam posted at 3:48 AM Fri, 18 November 2022 - #4449498

I need to upgrade to Big Sur etc - but after many attempts I haven't been able to get it. I'll try again, Poser 12 won't run on Mojave, which is my current OS. The most recent OS will not run on this iMac.

There is your problem.  Apple is leaving the PC realm and moving back to the bad old days of a walled garden, with very little software available to it.

Your choices are pretty simple - move to Apple Silicon (good luck with that), or move to a PC.

Move to the dark side - we have both cookies and performance (for a fraction of the price of the mac).  It isn't until after you leave that you really understand how much performance you gave up.  I am still kicking myself for not leaving in 2017, instead of waiting on Timmy & Sir I don't understand the laws of thermodynamics to deliver a replacement for the 5,1 Mac Pro.