I started out with my existing PAO system before later upgrading to double letter speffz; however, you want to keep the A-X logic that speffz provides, so that you don’t have to relearn if and when you decide to go back to speffz. Have a look here:
Using braille to transform letters into numbers has the advantage that a) you learn a new skill along the way; b) you keep the speffz naming intact; and c) you can practice the system by turning on braille input on your phone and just write some text.
Also, like you mentioned, do PA or PO in each location so that it’s similar to double letter pairs. By the way, I’d start with corners instead of edges, so if you want to go from Old Pochmann for both to OP for corners and M2 for edges, you just have to rework the part after the parity fix.
For just learning the memorization part you can also have a look at this app:
https://itunes.apple.com/us/app/memocube/id1236599758