We did all those technical bits like SPF and DKIM, put the one click unsubscribe link on the message and also on the header of the message so that clients like Gmail can put the unsubscribe link on their own UI [2], all the recommended practices. I remember using the tool Mail Tester [3] and the results where all green.
We doesn't have issues with being marked as SPAM by Gmail/Outlook, and have average open rates of 50%, which is a lot higher than the industry standard who is around 20% [4].
We have a good UX and an ethical way to treat our users, like all users have to opt-in to their desired newsletters when creating an account (or choose to receive newsletters without creating an account), one-click unsubscribe link in big text at the bottom of each newsletter, but also a one-click-no-need-to-be-logged-in link to opt-out of all the newsletters that the user was subscribed, and more stuff like that.
But one thing that I think give us a lot of reputation for the Gmail algorithm, it was that we designed a feature that if the user doesn't opened a newseltter for about 3 months, we started to send the newsletters with an alert at the top saying like "Seems that you aren't reading this newsletter anymore, you will be automatically unsubscribed in 30[n counter] days. Click here to disable the auto-unsuscribe." (the disable the auto-unsuscribe also was for people who have images disabled and we can't track the openings, but is small percentage). So with that feature, we get sure that our users where engaged with the newsletters, and we have a system to avoid sending messages to "dead" emails, maintaining a fresh and healthy database of emails, and seems that Gmail/Outlook knows and likes that.
[2] https://www.sendinblue.com/blog/list-unsubscribe-header/
[3] https://www.mail-tester.com/
[4] https://mailchimp.com/resources/email-marketing-benchmarks/