I’ve been following this topic with interest as I have been using APRS
since it started in ‘92 and downloaded it from BBS’s! It sounds to me that
Bob Bruninga’s original APRS would suit your purpose fine. It is low
resolution and easy to use for messaging etc as that was one of his
original concepts
You can still get APRS876 from Bob’s site and then run it under QEMU on the
RPi.
Also wanted to say that Andrew is doing a great job meeting the various
constituencies within amateur radio with YAAC and many kudos to him for
doing this and also for free!
Bob AB2NE