I could just create a rule to never send any mail from {myself} to spam
That was going to be part of my first recommendation.
Can you try creating a filter that labels the email, but doesn't use "Never send it to spam" in the filter rules?
It's quite odd as I don't send any other mail from these accounts but to myself.
That could be part of the issue.
The largest email providers are not going to block each other often based purely on Sender Scores (IP reputation)
Gmail learns and acts based in part on how many good emails your other accounts send into Gmail, and positive actions such as if links in them are clicked or if you reply to the email.
If you simply open and delete them that may be logged as a negative action by Google.