I have some experience with some of these apps:

• open food/beauty facts: both have a long way to go IMO. The ui is very janky and lots of things don’t work. Open food facts seems to be a bit better but not much
• loop habit tracker: fantastic app, I use it every day, never saw a bug
• gadgetbridge: the ui seems rudimentary, but it has everything you need and it works really well. YMMV depending on which gadget you have though
• openscale: only used it for manual tracking. It’s very very basic but somehow I didn’t find a better alternative

If an app doesn’t support reproducible builds, the version you can download from F-Droid was built and signed by F-Droid, not by the dev

[…], from that point the app will be built by f-droid with their own digital signature.

This part of your comment is not quite true. One of the advantages of reproducible builds is that the app can be signed by the developer but fdroid can still verify that it has been built from the correct source code. You can check out the documentation here: https://f-droid.org/docs/Reproducible_Builds/