This is an interesting concept but I think the dumbphone aspect will make people expect something different. It seems like it's more a static digital minimalism setup that you cannot easily undo. I saw someone doing similar by setting up MDM for their iPhone so you couldn't install anything without sending updates from a laptop.

I prefer the MiniageOS method if it's quick enough to refresh when needed. When you say banking isn't supported, I have a couple of banking apps that work on my two phones (AOSP and Graphene), would those still work? There are also a lot of apps that kind of rely on a browser being available even if not accessible (Uber is one but that doesn't work without Google play so whatever, but WiFi captive portals are another). Is there support for that?