Hi Amanda,
One interesting thing I've found about email subject line lore is that a lot of opinions are often stated as facts and become sometimes become unduly recognized as best practices. I've been in the email marketing industry since the beginning and I've never found a better authority on subject lines than Dela Quist of Alchemy Worx. Instead of taking a common sense approach (which is often wrong), Dela rigorously tests approaches. He has written a lot about subject line practices. Here are a couple of examples: https://www.linkedin.com/pulse/key-effective-subject-lines-originality-find-out-how-dela-quist
https://www.linkedin.com/pulse/opened-your-email-subject-lines-still-drive-engagement-dela-quist
Definitely check out the Alchemy Worx website:
https://alchemyworx.com/
Also, Here is a community of venerable email marketers - you won't find better experience anywhere else:
https://www.onlyinfluencers.com/