Wger maybe?

Its got a self hosted back end but i cant recall if the app works standalone

There's also Simple that might do it

Both on fdroid

I had made a comment about that some time ago which I link it almost every time this question comes up:)

https://mander.xyz/comment/1704758

Fastnfitness and openscale (and energize) might be helpful:)

I don't know about Flexify but perhaps you can message the dev and ask them to add the functionality you want?

I use flexify. It's not perfect but it's good enough and looks awesome.

I guess it depends on what you are planning to do. Me for example: I don't really have a workout plan but train more as I feel with my goal being getting as much volume per big muscle group as I can (timewise). So I just use Track n Graph. It's also kind of my all purpose app for tracking stuff. That way I don't need many apps.

Liftlog is amazing!