I just found a Video that showed how to customize the Activity Data Screens via Connect App: https://www.youtube.com/watch?v=gcIu6xThXYM
Unfotunately this feature is not available on my app (latest Version 4.72) with my Garmin Fenix 6x Pro (latest Software Version 26.00).
The only thing I can do under "Devices - Activities, Apps & More - Activities & Apps" is to install or uninstall Activities itself, but no way to customize the data screens as shown in the Video.