Yes and no. The Tamer Pass itself is multi-platform, and any progress you make will be reflected in all the platforms you have linked, but the Novas previously purchased are linked to the platform where they were purchased.
A practical example: if you purchased a Nova bundle on Switch, the owned Novas will always appear in your Switch inventory, even if you play Temtem on Xbox as a secondary platform. However, the rest of the rewards are multi-platform and will be available on any platform you have linked.
Feathers are multi-platform.
If you unlink the accounts, the Premium content will remain in the platform where it was originally purchased.