That burner phone can still be traced back to you.

Depending on what that means, sure. Most likely scenario is that you get apprehended, can't discard the phone for whatever reason, and the cops search it. In that case yeah sure it can be traced back to you.

If you are careful and only use it for (careful) photography and maps (as OP requested), then I feel like it can't really be traced back to you if you discard it or give it to someone else etc, except maybe fingerprints and DNA (but cops likely won't have the resources to do that for everyone in the entire protest). And for situations like this it a digital camera or a paper map could be traced all the same.

All “feature phones” are running some form of Android

I'm not talking about "feature phones". Just a regular old midrange smartphone. E.g. the original Samsung Galaxy A-series.

That burner phone is likely going to require you to sign into a Google account with a phone number before it will do absolutely anything

Some of them will, sure. Check before buying (or just make a new Google account without linking it to a phone number - might require a VPN to somewhere else but doable). I've had plenty of smartphones where you can just skip it. You won't have access to Google Play and such but that might be a bonus :)

Android requires this in their ToS

I'm too lazy to check but I highly doubt it - Google is not available in China at all and yet there are plenty of Android smartphones sold there. Also Android is mostly open-source and different vendors can and do build different versions of it.