Done, thanks!
Some feedback: you should add a PTR record (reverse DNS) for studer.dev and mail.studer.dev.
I also noted that your DMARC record has p=quarantine, which depending on the details of your outgoing setup, may cause your emails to be placed in the spam folder if DMARC fails for any reason. I generally advise against using DMARC with any policy other than `v=DMARC1; p=none` or else you're liable to run into issues, it's not a very good standard.
Otherwise, your configuration looks good to me. Also try https://www.mail-tester.com to see what they think.
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...