The best, and only, solution I’m aware of for syncing and using a TrainingPeaks training plan on the Apple watch is with the Stryd App. This requires the Stryd power device as well.
The athletes who use this combination report that they love it, that it’s super easy to use. And, you don’t actually need TrainingPeaks Premium for it to work. The Free Basic TP will still sync to the Apple Watch.
Because we only sell our plans on TrainingPeaks (due to a lucrative exclusivity agreement!) I don’t know about other training plan platforms and the Apple Watch.