My rational is that I am on a 2008 MacBook White. Originally, I had a 2006 MacBook Black and a 2008 MacBook White which were in various states of disrepair. They’d been sitting in my closet at my parents house for like… 6 years? I had my parents mail them to me because I wanted to sell my 2012 13" MBP to cover rent while I saved up for something else. In the mean time, because I have a Mac Pro as my main editing machine, what I really needed was your typical school/work/media consumption laptop. I thought I could either deal with an old version of OS X or with Linux.
I was able to bring the MacBook Black back to life with Linux Mint 18.3 32bit, and I suspect it will handle 19 32bit (iirc, it is NOT a 64 bit processor) just fine. I was able to get Snow Leopard on it, but nothing past that. Compared to Linux Mint 18.3 Sylvia, Snow Leo was ancient and most services wouldn’t work on it, it was hell finding a compatible browser, etc. It just didn’t do much at all in the modern world.
The MacBook White had some parts working, some not. It only supports up to Lion 10.7.5. Also just not good enough. I ordered another MacBook White for about $100 that was supposedly missing some parts. Its ram was corroded and its battery is now dead, but the ram and battery from the other MacBook White and a couple of other internal parts, and this is the computer I’ve done this write up on now. Before, I had two (potentially three) computers headed towards the dump, now I have one very solid office tasks machine (the MBB) and one solid machine for everything EXCEPT high level video editing and photo editing. And I have quite a few spare bits and pieces that can be used on both of them if I need them.
In your case, I would continue to use El Cap if you are not concerned about security and it still “just works.” I couldn’t (yet) put my main computer, the Mac Pro, on Linux because I’d have to completely change my workflow for video editing and photo editing, and it’s not worth it, even though my Mac Pro is now cut off from Mojave and will be stuck on High Sierra.
However, at some point, Linux setup and workflow changes, especially for you, MAY be a change that is worth it to keep a modern operating system on your machine. The question is, what do you do with the iMac? If it isn’t specialised, then maybe you can suffer the temporary immediate pain for the gain of a much longer life of your machine?