Use Thunderbird to download and search your email locally. Proton can't access the contents of your email so the search is bad.
Discussion
This is also good if you're worried about them deleting stuff
Wow, ok, I didn't realize that was tge reason. I'll give it a go
You can also make things easier to search by putting keywords in the subject, because that's one part that isn't encrypted: https://proton.me/support/does-protonmail-encrypt-email-subjects