After all, I can be one hundred percent certain that I am not the only one who has eight DPTRs at his disposal
No, in my 66x work I have two and the rule is:
save AUXR(data pointer select SFR)
save any dptr(s) the subroutine uses
restore on exit.

I do not think a 'selective use' of more than, say, 4 of your DPTRs will make sense. If I had 8 DPTRs I would probably assign 4 to the serial ring buffers (store and fetch) and use the remaining 4 with a rule similar to the one I apply to the 2.

Erik