Can’t help with OS install advice, but 2012 iMacs can still run Mojave. They’re dirt cheap these days too, often under $300 on eBay with free returns. While I wouldn’t want to run an 8yo machine, if given the choice I’d vastly prefer to run it than a 10yo one on an antiquated OS version.
I put a bigger SSD in my 2011 11" MBA. However when I upgraded to Mojave, which was an allowed update, it failed due to the SSD overheating. My reseller recommended not going above High Sierra for that machine. I passed this as a temporary measure on to a family member whose own old MBP died due to a logic board fault, with a stern warning not to trust it.
My 2010 iMac still had a fusion drive, and was beachballing all the time. I considered upgrading to an SSD but the machine would still have had primitive USB speeds and primitive Bluetooth which did not support handing over documents between Mac OS and mobile devices. Since I do photo and video editing, I wanted faster I/O. I changed to a 15" MBP a year ago, and am happy to be able to take this Mac with me when I travel. I have to keep the iMac as a firewire interface for now