If you're interested in extending the service life of that old iMac, I recommend OpenCore Legacy Patcher. It lets you run newer versions of macOS on old hardware.
I did install a more recent version of MacOS on the thing but only to see if I could since I do not use MacOS, preferring Linux or, to be more specific, 'Debian' on all my systems. Performance is much better with Linux, I do not need to fight the vendor who wishes for me to buy new hardware. With Linux the thing does what I want, when I want it without telling any third party about my activities. I also have a Macbook Air ('keyboard broken and display problems' which were quickly fixed) on which I run Debian, the same is true there.
https://dortania.github.io/OpenCore-Legacy-Patcher/MODELS.ht...