[Bangle.js 2] Seemingly random freezes / unresponsive?

I'd say definitely try a different clock face, but it could also be related to apps with boot code/widgets.

intermittently gives me a "DISK" warning

How often does that happen? That would imply that some app is writing to Storage very frequently (it'd have to in order to use up 8MB of memory so quick). And that could well be an issue.

When storage gets too low, a 'compact' pass is run automatically. As I'm sure you know when you've run it manually, it can take quite a while - could it be that's what is happening in your case and the Bangle is just sitting there unresponsive for ~90 seconds or so while it is compacting itself?

I'm connected to the BJS fork of Gadgetbridge. No weather or other online apps set up so far.

When it happens, please could you go to Debug, then Fetch Debug Logs. It'll write a file into Internal Storage/Android/data/com.espruino.gadgetbridge.banglejs/files (which you could easily see if you plug your phone into a computer) - and maybe you could see if there were any interesting-looking messages shown in that? I wouldn't post the complete thing up as it might include the content of your notifications though.

I sometimes see similar behavior. Mostly when connected to Gadgetbridge and listening to music/podcasts for at least half an hour.

That is interesting - I'll give it a try here and see if I can reproduce. Next time it happens, it'd be great if you could check the log to see if there's anything in it.