You did ask about "best MacOS" for your setup.
I would say that Mojave is best choice at this time, as Catalina has amazing issues for some users.
Because of that, I don't yet recommend Catalina, unless you know what you are getting into.
But, in the end it's your choice.
(I have a similar-aged system - a 2012 mini. I have a lot of experience with Macs - almost 30 years at a service tech level. This is my home system, and the first time I have not updated immediately to newest system. I do have Catalina installed on another drive, and test it out often, but I stay with Mojave until I see a few things improve. Such as right now, I am having what I call "tinfoil-hat issues". The mouse cursor suddenly runs around in random movements, and random typing "pauses"... I don't have a clue, does it a couple of times each day, randomly. Not a trackpad, but a wired USB mouse. Doesn't happen on my main system on Mojave, all with the same hardware. Looks just like someone else is controlling my system, for a few brief moments. Goes away after 15 to 30 seconds. Never seen the like on my own system.)
---Stay with Mojave for now. Just my 2 cents...