I'll cite one aspect of its design and implementation as a problem – it doesn't support standard protocols like IMAP unless you choose a paid plan. Even then, you have to use a "bridge application". On mobile, only the official app can access the mailbox. To sum it up, if you're on a free plan on ProtonMail, you cannot export your mails out (unless you do it by "printing" mails one by one). To contrast this, even "free" platforms like Gmail and Yahoo provide IMAP access to all accounts.
But that's the point of ProtonMail. All your data is invisible to them. If you could use IMAP without a bridge app, which encrypts and decrypts all your mail data locally, guys at Proton could read your mails. Not necessarily because they would want to. That's just how email works.
You can encrypt those emails with PGP, the best encryption there is.
A good email client can also learn to automatically encrypt when you send to a specific email, so you can send to a specific alias (with a plus or subdomain aliasing scheme).
But then will your diary service work right with it? Honestly, you should use a mail server that isn't from an ad company. And then if you also want to layer encryption on top, feel free.
(Also, calling PGP "the best encryption there is" might be a bit of hyperbole.)
It’s as private as your email...which probably means not private.
Like most people, I depend on an advertising company to host my emails. But I wouldn’t share my private diary with them.