I’m not really sure how it opens up replay attacks

Put simply, jt allows an attacker with a leaked database to use the hashed password as a password. In your original comment, it seemed like you were suggesting hashing only before transmission, on the client; but hashing both before and after would indeed patch that particular vulnerability. I don’t know if there are potential problems with that strategy or not.

another approach of client side decryption is to handle decryption completely client site

Here’s potentially an opportunity for me to learn: how does such a service (like Proton Mail) perform this in a web browser without having access to the data necessary to decrypt all of the data it’s sending? Since you can’t count on a web browser to have the private key, do you send down an encrypted private key that can only be decrypted with the user’s password? Is there some other way to do this that I’m not aware of?