I have everything aggregated into Gmail, so I just use web and the mobile app. I'm looking at Proton but it doesn't have the "send as" feature for external SMTP services the Gmail does.
I'm honestly a bit surprised that Proton doesn't seem to have the send as feature. I was able to find at least 15 posts across their uservoice.com site and their Reddit forum, spanning at least 6 years, with one of the uservoice posts having over 300 votes. I just gathered up all the links and sent it into Proton Mail support. Hopefully having all that thrown at them in one big bundle will prompt their project managers to consider it.
What do you mean? I use a custom domain in proton so that my server sends emails from info@mydomain instead of my proton email. I have about 10 addresses with three different domains in the essentials plan and I can send emails from different containers as different emails
That's only for a personal domain that you own and can set the DNS records for. But if you wanted to forward your gmail (or yahoo, or outlook, or whatever other provider that offers a public SMTP server) addresses to your proton mailbox, and be able send emails as those gmail addresses from within your proton mailbox, that's not supported. See here for what the feature looks like in GMail.