I have everything properly set up, and wasn’t blacklisted once in any service since 2002.... until 3 months ago, when Google decided to blacklist my domain, but only randomly (e.g. mails sent to same person on same day get delivered to inbox or spam seemingly based on a coin toss).

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.

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.

Well, you're right, though I've seen data in there for domains with about 10k emails per month.

Still, that's probably way over the home email setup threshold.