I think about how easily I could spin up a mint backed by 0 Bitcoin and I get concerned.

If every person that zapped you used a different mint and you swapped every zap to your mint, it would be no different fee wise than it is today. But in reality, people will coalesce around mints. They have less centralization forces than custodial lightning wallets, but will still tend to centralize. Because of this you won't swap individual zaps. You'll probably only swap once your untrusted mint balance hits a certain threshold. This logic could even be baked right into clients. Because of this, you will end up saving on fees.

Consider about how quickly this fraud would be noticed. The second anyone tried swapping you'd be caught and your mint would be ostracized by the community.

you could just use one custodian that you trust and you should be personally ok. you can choose, that's the idea.