Bluetooth settings (bangle js)

Hi, I think this answers most of your questions:

https://github.com/espruino/BangleApps/blob/master/apps/setting/README.md

(it's available by clicking 'read more' next to the settings app on https://banglejs.com/apps/)

But...

Make Connectable exists because we found people were having trouble enabling Bluetooth and turning programmable on. It's just an easy 'make it work' button, that can also work temporarily.

BLE on and Programmable off seems to work with GadgetBridge. But, when GB loses connection, can other people still connect and read data?

Not really. All they can do is issue the kind of commands Gadgetbridge does - so with a decent amount of effort someone skilled could get your heart rate or issue a notification on the watch - so it's not a massive security issue.

Which are the recommended settings

Honestly, I'd say 'programmable off' should really be good enough. Passkey pairing could be worth a go as I believe that can stop other devices from connecting at all, but it hasn't had a bunch of use.