Don’t worry about e2ee: Even if you get the most expensive plan from e.g. Proton, it’s not e2ee unless both parties use Proton. There is a free, “easy” way to realize true e2e: OpenPGP in Thunderbird (convenient), GnuPGP (more secure), etc.

As for mailbox.org: I used it before but it showed Google reCaptcha, which was an obvious red flag:
cf. [Security and GDPR Issue] ProtonMail includes Google Recaptcha for Login, every single time. #242

Also, technical score of mailbox.org has been relatively low, not improving: internet.nl/mail/mailbox.org/1080449/ (Don’t worry too much about this score, though. It’s only technical; human factors (philosophies, trust, etc.) are more important when it comes to privacy.) This is not a recommendation. DYOR; ultimately, believe your own intuition.