If I were you, I would not use any kind of non open source and non self-hosted email service pretending to be "secret", in the best (!) case it has some sort of

If I were you, I would not use any kind of non open source and non self-hosted email service pretending to be "secret", in the best (!) case it has some sort of silent metadata/access logging. While common shady services like Protonmail bluntly store plain text archives, and even if they claim they don't, there's no zero-knowledge proof on this highly sensitive topic.