Bangle2.js GPS performance

and the AGPS data expires within 6 hours

Are you sure about this? I was pretty sure the data was 48 hours (but it's not updated that often so realistically any upload would be valid for 24)

I thought uploading the data was only supposed to decrease time to fix, but it looks like it somehow determines whether the sketchier satellites will be visible at all

On Bangle.js 2 the upload also configures what the GPS is searching for (with the radio buttons). That's something that could be done with an app too, but it just made sense to stick it in the AGPS upload.

If you're not also searching for Galileo and Beidu then it can use more resources to search for GPS, which is why it can see more.

... but yes, sadly the Bangle.js 2 GPS isn't quite as good as the Bangle.js 1. Part of that is the chipset, but part of that is also that the device itself is a lot smaller and so there's not as much room for a good aerial.