>I've been running my own mail services for close to 30 years now...

There's the rub. 30 years ago this was true. Old systems have been grandfathered in.

A combination of rising spam and things like fraud via email have caused especially large services to be so much more aggressive in blocking. If your email has been around forever it's generally trusted.

The company I work for has been around for 15 years and we spent the first 5 or so getting yahoo and live/hotmail/outlook to accept our mail reliably despite proper dns/dkim/spf.

Self hosted on residential IP today is near impossible. Your only hope is pay to not be on a residential IP and even then strap in for years of struggle to get the biggest free providers to accept you as legitimate. Exacerbated by their thorough lack of actual support contacts.