Premiere Pro Workflow with Aging Mac and 6K Footage

and an ssd for your boot drive and a separate ssd for your media will also help. How old is your macpro?  I've got a 2009 macpro that's still cranking away.  Had to upgrade to high sierra to be able to run premiere 2020 which was a little tricky (apple doesn't make it easy as if you try and upgrade it, it will tell you high sierra is not compatible with the 2009 macpro - not true, you just need to install high sierra on a drive on another system and install the drive in the macpro).  How much ram do you have installed?  It's cheap and easy to upgrade the ram.  I've been able to edit a multicam project with 4 streams of 4k footage using proxies successfully.  

You'd definitely want to work with proxies, and the benefit there is also since they are low bitrate you can likely get away with a lower speed drive situation. You may not need a NAS - though obviously if that's in scope it's not like anybody would discourage a NAS or DAS situation. DAS would be cheaper, and as good or better speeds depending on your network situation.