I have couple of ideas:
1. Make data sync free in Windows 10 tablet and PC devices and request purchase only from Windows 10 mobile and iOS/Android users. Most users will use this app in their phone devices anyway. I guess this cannot break any Store rules (I can decide pricing of in-app products)?
2. When user purchases the cloud sync feature (using any of the Stores Apple/Google/Microsoft), save the license status in Azure database. Before initiating the data sync from any supported platforms, just check the license status from Azure and allow syncing if license is valid. But I am not sure if this brakes some Store rules.
What is the best way to implement this? Do you know any apps that have implemented similar feature?