I went with Fastmail for catchall because since the last time I checked (about a month ago) they still didn't support it fully. I hope they do support it fully in the future as I'd seriously consider switching.

ProtonMail now supports catch-all email for ProtonMail Professional and Visionary plans. That means for each domain that has email hosted at ProtonMail, you can now designate one email address as the catch-all address by going to Settings –> Domains. For example, [email protected] could receive any email sent to the @me.com domain, even if the address did not exist. You can learn more about catch-all emails here (https://protonmail.com/support/knowledge-base/catch-all/).

https://protonmail.com/blog/protonmail-v3-10-release-notes/

That's great to hear.

I'll probably have my current mail service run out and then switch to protonmail for personal mail and selfhosted mail for spamming notifications from my nextcloud instance.

Can you send email back from the addresses the email was sent to? I think when I was looking into it, ProtonMail caps how many aliases you can create; making it not that good for catchall.

What part prevents you from sending it from any email address? I assume if you smtp auth with them ok then you should be able to send from any email@authed domain you want.

Do people limit that somehow?