I posted a while ago, since then I DID upgrade my old iMac to Sierra - 10.12.6 (16G2136) *This just got an update a few days ago in fact, first time in a while..security update. All is well with it. I DID have issues backing it up on Time Machine to my old HDD as a precaution, for some reason it just saw and hung there "preparing", never updating. I had to do a copy using a program called "Superduper" - Took a LONG time but it worked.
Sierra installed fine. I dont need High Sierra.
My newer 2017 iMac I picked up in June came with High Sierra, I upgraded to Mojave. As soon as I did that it felt a tad buggy, apps crashed. The CPU rebooted a few times for no reason (I felt "great, brand new'ish iMac for me is a mess!".)
I ran disk utility by rebooting (CMD+R), this found quite a few issues with the volume after the Mojave update. Thankfully this seemed to fix it.
I am cautiously optimistic to update to Catalina on this 2017 iMac...but I will no doubt wait at least 2-3 weeks until I do that. Waiting 2-3 days before IOS issues, I need to wait much longer and see what issues arise when it comes to a new MacOS.