I recall the encrypted email providers were going to allow 3rd party audits. Using ProtonMail as an example, it looks like they're choosing to make code open source or limiting 3rd party audits:
We maintain the OpenPGPjs encryption library. After some recent code enhancements, OpenPGPjs underwent an independent security audit, the results of which are discussed in this article.
Recently, a self-published paper that was not peer reviewed claimed weaknesses in Proton Mail's cryptographic architecture. The claims made in this paper are not inherent to Proton Mail's architecture. We do not believe the paper’s arguments constitute "serious shortcomings," as we explain below.
protonmail.com
Is anyone aware of a provider that allows 3rd party verification and/or published enough details to satisfy their users with these concerns?
If you really think your "let's have lunch on Friday" messages deserve ultimate protection from prying eyes, encrypt them using an algorithm you trust.