OP's decision to move to air-gapped device is wise and he can't go wrong with it.
I mean, if we want to be pedantic, things can go wrong even in an air-gapped device. There may be hardware backdoors, OS zero days, perhaps the BIOS is compromised. Nothing is 100% secure, but if you migrate to an air-gapped environment, you definitely have less things to account for.
That's true most of the Operating System disable default Bluetooth by default but sometimes the users enable it and don't really care much about it because of the short range it has. Some people enable to it connect Bluetooth speakers and headphones and don't really care much about security when it comes to Bluetooth connection and that's can make things easier for hackers.
I don't understand how things can any worse with bluetooth than the Internet. First things first, the user must approve the wireless connection, it's the default security on Linux AFAIK.