Protonmail is definitely more private than google or Microsoft, but you shouldn’t hold 100% trust in any provider. Ultimately your data is still on their hardware and they have control of it. Also, as others have pointed out, both sides need to be secure otherwise all that data is accessible on the other side.
You can mitigate it yourself a bit by hosting your own email server, but I highly recommend against that as its a massive headache to secure and basically every provider will reject your messages anyway.
“Be happier or it’s another hour in the humbler”