And then one spend merges them all into one transaction and then suddenly everyone knows your past.

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.

The concept of a SP wallet is different. A holder of a SP wallet is an observer of transactions. Once they identify transactions that are theirs, they should sweep to a fresh address.

We can track the fresh address to. It's not that they need to sweep into a fresh address, it's how they perform that sweep that matters. Remember that we can also see and track these "fresh addresses". It doesn't take much to bundle them all together.

Every address can be tracked. That’s how UTXOs work. The issue is whether the address can be attributed to an entity. That’s why wallets have coin selection to help mitigate this risk.

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.