The Visionary Plan should have the option to add more paid users.
For example I signed up ad few years ago for the Visionary Plan and then I register my custom domain because I’m creating my own company and of course all companies start with a few couple of members. Now that I have used all 6 users I just need 1 or 2 more users and the Proton Support team is just telling me that the Visionary plan is not customizable and upgrading to a Mail Essential will be more expensive that having a Visionary Plan.
I just want to pay for 1 or 2 more user while keeping my Visionary account under the same custom domain.