Part of privacy is reliability and certainty. Just take people's money like Proton, you can still have a free tier.

You have "email protection" but do you have nice stuff like sandbox detonation and private intel partnerships with companies like Proofpoint? That as a premium might be worth charging. Heck, even public yara rule scanning and allowing inbox owners to set things like DMARC enforcement and custom rules and actions would be a nice premium.