I've been running my mail server for a while by now.

In my experience Gmail isn't a big problem. I mean, it's quite strict about DKIM/DMARC/SPF/RDNS stuff, it wants to wait a bit before trusting a new server, and one single mistake is enough to get you blacklisted for a while. But if you know your 21st century postmaster fundamentals, and you aren't on a Spamhaus list, you can get your mails delivered to Gmail 99.9% of the times.

The real problem out there is Microsoft IMHO. Even complying with all the best practices out there often only results in them "temporarily whitelisting" your domain. Microsoft has sent even seasoned postmasters to the hell and back with their illogical policies. And of course their support is a complete black hole - the only feedback I got once for them, after asking why they would periodically blacklist one of my domains, despite having good reputation and no issues with any other mail services, was that "they can't disclose their acceptance policies with other postmasters".

Microsoft is the real reason why people who would have all the skills to set up and run their own mail servers eventually have to resort to paying money for SMTP relays like Mailhop, Mailgun, Proton, Tutanota etc. - just to have the piece of mind that Outlook/Hotmail addresses won't start rejecting emails out of the blue. SMTP relays are literally the "Microsoft tax" for daring to run your own mail server.