Cannot connect banglejs 2, also cannot access menu screen

Using the HRF Connect app on Android, it shows "Connected Not Bonded". The first time I used the app, it disconnected after a few seconds. Now, it stays connected.

Well that's pretty promising then. What device name were you connecting to? DfuTarg (DFU) or the actual Bangle.js abcd? I think the DFU may automatically reboot if the DFU process hasn't started within a certain time, so that could be why you saw the disconnect.

Since then, it would connect to GadgetBridge but not the apps page.

This is what you'd expect normally... The Bangle can only have one connection at a time, so if you're connected to Gadgetbridge, you'd have to use the apps page within Gadgetbridge. The one on banglejs.com/apps won't work unless you explicitly disconnect the Bangle from Gadgetbridge (and if you have 2 phones, both of which have Gadgetbridge, when you disconnect one, the other may automatically reconnect?).

Have you also tried just rebooting (or turning bluetooth off and on again) on the phone/device you're having trouble connecting from? It actually seems to be quite a regular occurrence that the bluetooth radio can get confused on some of devices (I had to do it on my PC just this morning).