Couple things that I’ve found out,

• Gmail seems to need your server to have IPv6 with PTR, even if the mail is sent over IPv4
• Even a DMARC record with no ruf or rua helps lower the spam score
• For Outlook you need to send some mail to yourself or someone else and mark the messages as not spam manually for a while
• MS365 will even put mails from Gmail to spam initially
• Some TLDs like .xyz will go to spam even if everything is set up perfectly
• Outlook also seems to cache DNS quite long, you may need to wait a day for changes to propagate
• A recently registered domain will land in spam more easily, if it has been registered for a while it also seems to help

If you’re not already familiar with these, https://mxtoolbox.com/SuperTool.aspx (write smtp:your.mx.record is a good tool, and I’ve also used https://www.mail-tester.com/. Mxtoolbox blacklist check is also good.

I hate it that spammers have made hosting email such a hassle. Hope you get stuff running!