If you don't like the interface, just use your own email client.

Proton supports POP/IMAP. If you don't know what that is, it basically is a protocol that allows a mail server to talk to your computer. This allows you to use whatever email client you wish, such as Outlook or Thunderbird. As long as you can configure it to talk to Proton's servers, and you download their "interpreter" Bridge (necessary because of the encryption), it will download all your mail to your computer and then you can configure it to your heart's content. This includes custom domains.

I strongly prefer encryption because, unlike Google, Yahoo, and the like, they can't read your emails. This is a risk for you and your clients.

I have been using Proton for my business email for a few years now and the only "complaint" I have is that the Bridge can be a little slow -- especially with attachments. But this is to be expected because it's encrypting/decrypting a lot of data. I find it acceptable because, again, the trade-off is very much worth it for me.