haven't you heard you're supposed to never sell or spend bitcoin?

If done right, it breaks the link between sender and receiver. The receiver still has some downstream transaction risk, but that’s on them. I am more concerned about protecting the sender. This applies to all Silent Payments - not just Nostr Silent Funds Wallet.

Not after a bad spend... On a donation portal, for instance, it would link all sender's as soon as the receiver makes a spend that uses coins from many if not all at the same time. It's even worse if the receiver is constantly spending from the SP wallets.

For the donation example, I think it's only an issue if the the SP addresses are linked to their original public static address with a consolidation, otherwise none of their other silent addresses are linked to anyone. They might be inferred if they end up paying to a particular static address. But the more silent payments are used in the ecosystem then the less traceable it all is.