Have you tried to check https://postmaster.google.com/? You _may_ get an insight on what went wrong those 3 months ago
That requires hundreds or thousands of emails from your domain per day to show anything - which is useful if you run a major service, but for the use case of "every person hosts their own email", it’s not exactly useful.
The only clue I have so far is that in some discussion, it was mentioned that google penalizes a domain of a lot of different emails from it get forwarded to google - and I do have my catchall forwarded to a gmail box.
We now live in a world where google can make you and your communication channels disappear, and they don’t really answer to anyone.