Sooo - iOS 14.0.1 and WatchOS 7.0.1 did not resolve the issue. However, I noticed a support article from Apple: https://support.apple.com/de-de/HT211865
That actually helped.
For the most part at least: new outdoor workouts are now properly recorded again and battery usage is at the same level as with WatchOS 6.
Previous workouts done before the iOS / WatchOS upgrades still don’t show routes though. I guess I’ll have to stick with that and won’t get back my previous recordings.
That reminds me: why doesn’t Apple add a GPX export in the share sheet for outdoor workouts? As long as they don’t have it, I still need to use 3rd party apps in parallel. So sad! ;-)