I selfhost on a cheap VPS I found on LowEndBox. The security setup (not SimpleLogin) was more involved than SimpleLogin. Pretty easy to get working and I’ve had not a single issue in 1.5 years of selfhosting it. I pay $11/yr for the VPS compared to $30/yr for SimpleLogin.

With regard to your specific questions - I’ve only selfhosted for myself with one domain. You can add multiple users and domains though.