As well as my 7,1, I still have a perfectly working 2010 5,1. And, there is no reason to upgrade the OS beyond Mojave. Evrything since then is mostly about Apple control, their own walled garden, T2 chip, shareholders, lack of user control etc. The 5,1 is certainly not a museum piece and certainly continues to work well for pro audio, film not so much becuase of the lack of GPU grunt. In pro audio, all of the apps, drivers, DAWs etc continue to work very well & without all that so-called 'security' BS, eg, at various times recently when Apple servers went down and was revealed just to what extent Apple spies on all of our installs. Then there's the more recent bricking the hardware and needing to recover by using yet another Apple mac and specialised software etc.
A clean 5,1 + Mojave is still a very nice setup indeed.