@mrjoshjohnsmith just FYI, Android phones can still be connected via WiFi and used for things like the apps without having to have it connected to mobile data or anything. In other words, all service can be disconnected to the phone and it will be able to be used for anything but text messages and phone calls. (Though honestly, there are free aps for phone calls and texting via Wifi).
So you could/would use your Android 6 only as a controller for the bed and then would have your better/newer phone for your personal use. By no means trying to force you to it, but wanted to make sure that was on your mind as you’re trying to say you can’t update to a newer phone because of the one app you need.