If you make a list given the criteria in this post how many contacts appear on it?
Ultimately, if you stop sending to whatever amount of contacts that is (which have an ROI of virtually 0) your overall open rate will rise, and you will minimize spam folder placement and/or bouncing due to graymail.
Also, DKIM is a no brainer. In your post you mentioned that MXtoolbox failed DKIM. Please ensure that you have set up both the TXT and the two CNAME records for proper DKIM authentication, and that those values match what is in your domain manager exactly https://knowledge.hubspot.com/articles/kcs_article/cos-general/what-is-the-updated-process-for-conne…