The contents of proton pass are end to end encrypted
That's precisely why they can't do the scan from their servers. They don't know the domains - they're decrypted only on your device.
this has caused my whole vault to be leaked to public DNS servers via unencrypted UDP
I feel like this is a tad dramatic. Surely, your vault contains more data, probably more sensitive, than just domain names.
I have never tried Swisscows but I just want to say that Kagi is fantastic. Absolutely the best search experience I had in years.