Some Qs on security/safety/precaution measures about BJv2 thx

I could get on a train, and use DaFit to brick every other's DaFit smart watches if I wish

If the watch wasn't already connected to someone's phone then maybe. On Bangle.js you can add Pin pairing, whitelist, or turn off Bluetooth totally though.

BJv2 wont be encrypted as full-disk-encryption

No - while technically possible to do, realistically the chances of someone opening your Bangle, reading out the flash reverse-engineering Espruino's filesystem really is quite small. I think you're in danger of tin-foil-hat wearing security paranoia.

But of could someone could (once they have physical access to your watch) change bluetooth settings and load the data out with the IDE/app loader.

thus if I wanna develop something sensitive, like a cipher or password manager, the program it self should be zero-knowledge, until the user feed some keys into it?

Yes and no - I mean, if you encrypt the data with a PIN code that the user has to enter, I don't see a big problem there. There is AES encryption built into Bangle.js so you can easily encrypt individual files if needed.

Of course someone could brute-force a PIN but they would have to be able to find a way of knowing if the password was legit or not - but the same would apply to any device.

And yes, you wouldn't store unencrypted data - just keep it in RAM, or even better dispose of it as soon as you've drawn what you need to on the screen