Use a custodial keypair (C) for each. If it happens that at any point they decide to create their standalone keypair (S) outside of TIDAL, you allow them to "adopt" the custodial key from the standalone key using NIP-26 delegation.

If someone is following C, their client that supports NIP-26 should be able to recognize that S->C, so they should automatically associate both and also follow S.

After some time, the user should be able to exit TIDAL completely and just use S (although TIDAL could also let them export C just in case).

I am not sure there aren't problems with this approach, but sounds like it could work.