In the end I opted for a hard reset.
I went onto the General Setting in the watch and chose Reset Settings.
This deletes all the data on the watch, so I lost all the activities I wanted to keep (one doesn’t climb Mont Blanc every day ).
Afterwards, I deleted the app on the phone and started all over again.
Log in, link devices, and now it seems to sync correctly.
Just in case I’ve opted to keep Weather, Sun rise/set, Media control, and all other non essential features inactive.