Yea I ran into the same issue. I've got SPF, DKIM, DMAC, and reverse DNS for IPv4 and v6 and still get flagged as spam. I wrote about this a while back:
It's not as bad now, so I suspect someone at Google is finally listening, but I still don't trust the reliability of e-mail. If I sent someone an e-mail I haven't e-mailed before, I'll let them know on Hangouts/SMS/Facebook that I sent them an e-mail and to check their spam folder.
https://battlepenguin.com/tech/how-google-and-microsoft-made...
It's not as bad now, so I suspect someone at Google is finally listening, but I still don't trust the reliability of e-mail. If I sent someone an e-mail I haven't e-mailed before, I'll let them know on Hangouts/SMS/Facebook that I sent them an e-mail and to check their spam folder.