What types of apps are you missing from the FOSS ecosystem?

How about

f-droid.org

PlugBrain | F-Droid - Free and Open Source Android App Repository

PlugBrain blocks apps and uses math challenges to reduce screen time.

Not precisely what you’re looking for but maybe could be adapted.

Alternatively, how about setting a 20min timer when you start using the phone? It means you have to remember to start it as you open the phone but would remind you to take a break.

I hope you’ll take this in the spirit in which it’s intended.