If it is really a 2012 iMac (vs an older one purchased then), it can be upgraded to Mojave (and should have been a long time ago). Mojave will run AC 23 just fine.
Since you have not updated at all since 10.10 (5 version of the operating system have been released since then)... it may be a fear of updating? Be assured, when you upgrade your MacOS, it has not affect on your data files and settings - only the OS. Still, it is wise to do a complete backup before updating!
https://support.apple.com/en-us/HT210190
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB