Hi,
how should I handle planed vs. executed workouts in GC 3.8.? Sould I
remove the planed ones or can I manualy merge them together?
Now all
workouts are counted up. so I see mostly the dobbled miles, time, ... of
activity.

Wow, that looks nice I'm excited to use and test that view!
To make testing easier Joachim's PR is merged and new snapshot builds are available from https://github.com/GoldenCheetah/GoldenCheetah/releases/tag/snapshot, there is no download limit there.