I’m also a happy Migadu customer, and I have never had to interact with their support. I use it for most of my domains, except for a few that are work-related.
Ooh, looks like they added that in late 2023. Man. In August 2023 I actually migrated all of my email to Proton and was ready to go when I realized they didn't support forwarding. Thanks for letting me know.
Inbox. They claim that the inbox is special in IMAP and it's hard to have a lot of messages there. 150K messages in the whole mailbox, I think. 25 years of email.
The downside is that downloading messages is fairly slow when you have 10-20k messages in your inbox. And the webmail is fairly primitive.
I never tried Fastmail.