leave them spinning, uxto sets continue to gain entropy over time even if not chosen for a mix by maintaining the derivation path.

If you use sparrow wallet you can “mix to" any xpub you choose.

If you use Foundation device you can mix from sparrow to a Cold xpub created on Foundation AND keep the derivation path.

This means that when it is time to spend from cold storage you can still use the post mix spending tools of Samourai/Sparrow!