I like the security of Protonmail, but realized that its best use is when both sender and receiver are both using Protonmail, so messages can be fully end-to-end encrypted.

I moved my domain from G Suite Legacy to iCloud+ but kept a separate free Protonmail so other contacts who care about encryption can see the "@protonmail.com" email and message there.

With a custom domain, people wouldn't know if it's Protonmail encryption compatible without typing the email into the send message window first.