There are far too many moving parts to be specific here.
Start with the online email checks on mxtoolbox.com, internet.nl and mail-tester.com and see if everything passes there. And yes, for Google to accept your emails, pretty much everything should be “green”. ;-)
You should pay special attention to PTR Record, DKIM, DMARC, and SPF. Enabling MTA-STS can also help, but in my opinion, it’s not mandatory, and if set incorrectly, it can even be harmful.
And then, of course, make sure that your IP or domain is not on any spam block lists such as Spamhaus, etc. This can also be checked with MX toolbox and other online tools.
If everything looks good and the emails still won’t go through, you might also want to register with https://support.google.com/a/answer/9981691 to gain more insight.