I run CalyxOS on FP4, and I like it. It also has FP5 support. As far as I know, mobile Linux distros like postmarketOS work on (at least) FP4, but key phone functionality is lacking. There’s a functionality matrix on their wiki.

This is what I use. Replaced my old Fitbit Aria 2. I weighed in on both scales for about a month, and it was consistently 0.15 kg below, which is good. The body fat measurement was a bit more off, and it varies almost nothing over long periods of time, but I don’t really trust those measurements anyway.

I believe you can set up the scale in GadgetBridge as well, but I have not tried to do that.

I use ledger. I have not automated so much outside of autocomplete macros in my text editor, but it doesnt’t take too much time and forces me to look over my spend, so I like it. I will eventually attempt to build some kind of Dash-application for visualisation of the output, but have only started on the parsers so far.