Thanks for the hint. I have seen wger as well and it seems good. The only issue that I have with it that it only allows for workout routines where the same workout happens at the same weekday. E.g. it is hard to describe a workout with A/B splits in there (like r/Fitness Basic Beginner Routine).
I guess it will work fine though, since I am trying to move away from that workout anyways.
As for the rest in f-droid store ... flexify and massive seem to be fine, but the others seem a bit outdated looking at their latest release dates.
Thank you for the response. I was thinking about an actual app, since I hate working on spreadsheet on my phone, especially while in the gym.