Mailbox.org is a good pick to consider IMO. You can read some comparisons on PrivacyGuides, which I also recommend as a starting point for these sorts of topics. The mailbox.org web UI is not great, but it allows IMAP/SMTP access, so I use Thunderbird on both desktop and Android in order to interact with my inbox. My inbox is auto-encrypted with PGP using their Mailbox Guard thing, so my emails are all encrypted garbage on the web UI anyway. Mailbox.org only allows paid-for accounts, but considering the annoying stuff that Proton and Tuta do to their free accounts I'd rather just be honest about the service I'm getting. It allows auto-forwarding directly in the web UI, but given that you can hook up to it with IMAP anyway, it's not like you couldn't just do it yourself.

(Also, as another comment said I also recommend DuckDuckGo's Email Protection for email aliasing if you need it.)

Encrypted Private Email Recommendations - Privacy Guides

These email providers offer a great place to store your emails securely, and many offer interoperable OpenPGP encryption with other providers.

^{Privacy Guides}

Nothing is preventing you from changing those email addresses to the one you now have on your own domain.

The ignorance from OP is not vendor lock-in.

I think OP is overblowing things, and is especially misguided in recommending gmail, but at the same time, they do have a valid point and I think you're somewhat misrepresenting what they said.

For one, they specifically said that the proton domain email addresses are problematic (protonmail.com, pm.me), and weren't talking about custom domains that sit in front of Proton mail.

For two, their point is valid. Auto-forwarding being paid, does create vendor lock-in and make it hard to switch away from Protonmail if you use the OOTB addresses. It's something worth considering.

As you said, the recommendation should be to use a custom domain that sits in front of Protonmail rather than switching to Gmail, but paid auto-forwarding is a valid criticism.

I have selfhosted my mail on my own domain, on a server in my closet, for about 4 years. In that time I never switched over realy important things, like government etc, out of fear of missing an email and not knowing. That was the only reason not to switch for me.
I've had to move a few times in a short peroid and my career started taking more of my time, so I have less time to manage the server, thus I started looking around. Now I settled on proton, I don't use any of their domains, only my own, and I've switched literally everything over, except for the recovery mail for my domain registrar login.
Since proton hosts professionally, I trust them not to drop mails without telling me, and I don't realy see a reason not to switch over. If I ever want to move provider, or start selfhosting again, I'll setup the new provider, update the dns for my domain and done.

If you have some specific questions about my setup or choices, let me know, although I'm probably not able to reply in the next 12ish hours.

The straw that broke the camel's back for me is the CEO's icky tweet about how great Republicans are for your privacy and how they stand up for the little guys (what), which they doubled down on using the official Reddit Proton account. There's already been a ton of discussion about this on the internet if you care to look for more angles on it.

But before that I'd already grown quite leery of them for their trend of endlessly starting new services before the old ones are polished, along with trying to push everyone into their walled garden and endlessly using naggy popups in the UI about it. Worst of all, they have a clear trend of not giving a damn about Linux support, sometimes giving up on certain features for their Linux clients or releasing the clients way after the Windows/Mac versions. For a "privacy company", not putting Linux as a first-class citizen is really just unacceptable, and they've been around for long enough that it's clearly a trend and not a fluke. To me, Proton just feels like a wannabe version of Apple. Its continued actions give me the feeling that it exists to serve itself, not its users.