In case anyone is curious: I checked and the PTR record for my mail server (168.62.161.121 -> mail.studer.dev) is valid. mail-tester.com also gives me a 9.9/10, but I just tried sending an email to a Gmail account and it went straight to spam :(
I suspect it's probably a volume thing? As in I'm not sending enough email for Gmail to make any judgements about my mail server, since I'm only an individual. Of course there's not really a great way to resolve that...