Just pay for SimpleLogin no? Proton owns SimpleLogin now.
I purchased SimpleLogin before Proton purchased them. I have my own domain configured with all my aliases which all point to a proton email address which I do not give to anyone.
I purposely created my own domain just so I could be flexible in the future and move to another provider if needed.
I wanted to test sharing my calendar with my wife (we use Google which is currently how we share) but you have to have a paid account to share your proton calendar. I'm happy to pay but want to make sure it works before I do!