Hallo Mike!

 MP> Is there not a "pure 64 bit" raspbian distribution specifically
 MP> for the RPi3?

Not that I am aware of.  As far as I know about it raspbian is resisting 64 bit 
development in order to keep the distribution compatible with all the raspberry 
pi's up to and including the 3b+ which is the latest and greatest.

Het leven is goed,
Maurice

... Huil niet om mij, ik heb vi.
--- GNU bash, version 4.4.23(1)-release (x86_64-bonnell-linux-gnu)