Bangle.js 2 - Battery always 100%

It might be best to just stick with

battery level can be a bit erratic, as it depends on both temperature and current load

Filtering will never be perfect, and then you end up having to explain it anyway, except now complicated by the filtering.

And GadgetBridge could be a bit smarter about the battery level warnings, i.e. wait for the battery level to go above 40%, or the charger to be connected, before resetting the warning.