I'm obviously unaware how the filter makes its decisions to flag or not a message, but in my naive view of the world, I should have amassed quite a bit of positive signals, marking me as a legitimate user:

  - I've had an active account for 8+ years;
  - I'm a paying user (Google Play/YouTube);
  - I've linked my Android phone (with all the data that I gladly let them collect from me);
  - I've not used any add-ons to send mass emails (e.g. Gmass);
  - I've had 2FA turned on for years;
  - etc, etc.

If anyone from the Gmail team reads this and wants more input, I'm glad to help out. I love Gmail but it's slowly becoming less attractive (first the huge bloat - i.e. what inspired Superhuman, now the spam issue).