These are subjective opinions. Mailbox also has functionality that Proton doesn’t.
Proton is not fully open-source, their server is completely proprietary. If you use an open-source client like Thunderbird with mailbox.org the end result is practically the same.
Welcome to the forum! 
I think mailbox.org would be a suitable choice for you. It wouldn’t be recommended here if it weren’t a good service, and they have a very positive mission which you might value. You can also take a look at Privacy Guides’ recommended email clients which should resolve some of the UI concerns you have.