I give some input because I love the idea of send/receive via a special mobile device.
A lot has been written about different versions of Android and the access to external memory for apps, for users, for file-managers. Your question about “why” will lead you to many comments about Google and their evolving policies, and that is not so helpful.
We are at a site where we had a very expensive sat-modem running for a few years because the other option (long-distance dial-in to the capital) was even more costly and had appalling not-performance. I propose that such a special setup, like being off internet and needing to travel by boat etc. should turn the problem around:
Consider not to wait or hope or expect for good ole Paratext to cope with every special case of Android/Memory combination. Just recoice that there is now Paratext Lite and consider purchasing a dedicated PT-travel-phone (not expensive) (and a box of zip-lock-bags) which you have tested with a solid, reliable memory device (USB drive, card).
Again a personal testimony: When / if you manage to root your Android device, you have much better chances of being able to do what you want. Storing your data where you need it. Looking at all your memory. Allowing your apps to do their jobs.
Rooting is strong medication with side effects. But it would work for you, if you had a dedicated phone just for Paratext Lite. You could look at readiness for rooting before you purchase. And no banking app would refuse your phone. And not much personal data or other assets would be exposed to the often-mentioned risk of operating a rooted Android device.
Just some ideas. You guys are a reminder that West Africa has gotten cozy over the last decades… All the best. Please share anything that works for you.