FYI, there is an existing solution that does pretty much what you describe: https://play.google.com/store/apps/details?id=bsoule.tagtime...

Thanks. Interesting, looks like it's being developed by the Beeminder people. I probably would've given it a go if I found it (discoverability of PlayStore sucks), but now my 30-minutes solution is actually more feature-full than this (in particular, it works on a smartwatch).

Ooh, any way we could entice you to describe that in the TagTime forum? https://forum.beeminder.com/c/tagtime

So can anybody explain why did they decide to split Python into python3 and python3-pip packages?

Pip is not some random third-party library, it's part of the official installation that you can get from python.org.