Protonmail doesn't have unlimited addresses for sending, which is one of his requirements. It has catch-all , which means you can receive from an arbitary number, so it's okay for one -email-per-vendor, but if you ever need to send via one of those addresses then you have to use up one of your 10 or 20 allowed addresses.

This is because like most other vendors, they do price discrimination based on number of users. Which is no doubt why the other large vendors were also ruled out.

One vendor who doesn't do this is Migadu. I don't use them as they lack 2FA support, but they are one of a small number of businesses who make their proposition easy to evaluate quickly, as opposed to making you swipe through a pitch before you get to the facts. This is so unusual that I remember them despite evaluating them multiple years ago.