FOSS android user here (I'm on the HN Materialistic app now actually)!
I had the same issue with my bank, but I resolved it using the F-Droid app "Yalp Store", which wraps the playstore. Now I have my banking app on my phone!
It's not FOSS, but even on my Linux PC I have binary blobs for drivers and stuff. It's good enough for me, it might work for your app too