Questions:
"AFP is being discontinued late next year? Would we still be able to access external drives with that format?"
Consider yourself as having been duly warned about what's going to happen.
I'm going to GUESS that by OS 28 (perhaps even OS 27 next year) there will be "no access".
Time to get your network re-arranged with something else.
The sooner, the better.
(Remember the part about having been duly warned...)