Thanks so much for sharing your data. I learned very early on that most advertising spend for non-fiction isn't as powerful as it is for my fiction. I have 7 non-fiction books out, and run an average of $10 in ads a day for each. Those ads operate at break even, or profit, or they're killed.
I sold about $50,000 in non-fiction in 2018, and almost all of that was pure profit since 6 of the books were already out, and the 7th had less than $1,000 in production costs.
The reason I'm selling so many books is platform. The reason why Seth Godin can trigger a rush on your books is platform. Anything you can do to increase yours from podcast interviews to content marketing will increase yours. Given your reviews you've written the top book in your space, but your cover isn't doing you any favors and is part of the reason why your conversion is so low.
If you changed that one element I bet your profit from ad spend would sky rocket. Take a peek at my covers. The most important element is how it appears as a thumbnail. Note that all seven of mine have legible titles, bright eye-drawing colors, and can be easily read in the thumbnail.
Take some of your 20% profit and get the best cover in the business. Or, if you're still confident in your design abilities, learn what makes a great digital book cover and re-do yours. The thumbnail is everything.