Chances are they could detect it and ban you. Hell I got banned from discord for interacting with their API using my own authorization token for a normal user account... To be fair my custom client was not convincing at all, but oh well.
Try FairEmail, but it doesn't seem to have ProtonMail supported out the box and I don't know or really care how to set it up.
Discord...
Well the silver lining is that we will move to better platforms like Lemmy, Mastodon, Revolt, Signal... well whatsapp hasn't enshittified itself that much recently has it?
As a kid myself, I should not be agreeing. I've seen so many young people destroy internet spaces or just make everything worse... I'm just glad I wasn't part of that as much as other people may have been.
There is probably a way to age gate things well or whatever but I don't know.