Ask HN: Secure Email Encrypted at Rest

I am looking to ditch gmail. I have a superficial understanding of cyber sec/crypto but I am not an expert. That's why I'm asking this question of you.

I need an email provider that 1) Not only promises not to read my emails, but literally CANNOT read my messages due to underlying encryption, 2) Allows for searching of emails (some services cannot allow search when mail is encrypted) 3) Encrypted (at rest) calendar, and sync via calDAV, 4) Open source.

I have also used this handy tool, and when filtering on https://thatoneprivacysite.net/email-comparison/#detailed-email-comparison

When filtering on whether the platform is open source, calDAV support, and encryption at rest, I get only the following options: Posteo KolabNow Disroot Tutanota

I have researched historical HN discussions on a variety of email providers (runbox, fastmail, posteo, mailbox.org, tutanota, etc), and there have been some very scathing reviews of the encryption algorithms, methodologies, implementation, platform security, etc.

For someone that doesn't want to send encrypted mail, but wants a basic service where I can be confident the data stored is mostly safe* from surveillance and arbitrary access by anyone other than myself, what would be your suggestions?