Thanks for all the useful feedback. I should have been more specific with my original post.

The scenario is, funds were mixed using whirlpool and then sent back to offline wallet ("mix to external xpub"). The idea is to move those individual utxos to a multisig wallet.

Started out this way:

Singlesig (offline-wallet) --> whirlpool (hot-wallet) --> Singlesig (offline-wallet)

Question is what's the best strategy to now go to the desired end state:

Singlesig (offline-wallet) --> Multisig (offline-wallet)