Best OS for a Late 2009 21.5 in iMac?

I have a Late 2009 21.5 in iMac running OS 10.9.5, which I really like and is running smoothly. But Apple is phasing out support for the OS Xs, and already my iTunes is being very finicky, and dropbox is telling me that their desktop app soon won't support this operating system. I want to update the OS, but not to the latest one since it will probably bog down my computer. Do any of you have thoughts about what the best OS to run on this computer might be? I was thinking maybe one from 2013 or so.

I also have an iPad air from 2017 and an 80GB iPod from 2007, and I want to be able to manage both of them from this computer, which I realize is tricky, since apple isn't really expecting people to have peripheries from a decade apart. Already the iPad is too advanced for my OS and iTunes, but I don't want update my OS and iTunes to the point that I can't sync my precious iPod anymore.

Any thoughts would be very much appreciated!

***

[Edited by Host]