It's not that hard to use BTCPay server to generate new addresses for each donation, but problem could happen when you decide to consolidate all those transaction at some point.
Only way to keep some privacy would be to have good address management or to use some services like Chipmixer that would break connection with previous transactions.
Coinjoin could also be an option but it's less effective than mixers and they are easier to track with Chainalasys and similar tracking services.