That ideally shouldn't happen if your dkim, dmarc and spf check out, though. I hosted my own email for a couple of years and I can't remember a single time when my emails to my friends ended up in spam.
"ideally shouldn't happen" doesn't mean the deliverability cartel doesn't block you anyway.
Gmail et al have been spam filtering messages from correctly configured mail servers for a decade+ now. All the dkim, dmarc, and spf in the world won't help you if you aren't known to them.
Sure, that's almost certainly true, but the arbitrariness of changes and blacklisting leads one to consider perhaps they don't care at all about small hosting operations.
How are these HAM signals? Any spammer can set these things up. dkim/spf are just mildly useful anti-spoofing technologies.
Google and others will happily block your mail or send it to spam folder even if you never sent one SPAM email ever, and have all those technologies you mentioned set up.
I realized I wouldn't use it for anything serious, and I didn't renew my domain name. Maybe someday I'll get a domain for ten years and then get google to host the actual email. That way it doesn't matter too much if google decides to nuke my account.
I was also sixteen when I did that, so I mean, of course I wasn't going to do anything serious with it.
Unfortunate reality. I stopped self-hosting after many years three or four years ago. Keeping up with the requirements to stay out of the spam bin was too much work, and the risks of non-delivery were too high.
Remember, even if you have everything 100% nailed down with SPF, DKIM, etc. you can still end up on a random blacklist, some of which are basically extortion shakedowns. Now, you can say, "well ignore those losers, who cares?" but sometimes you have a customer who directly or indirectly relies on those blacklists. I certainly do, that's how I found out!