How do you get more user feedback on your content?

Hey everybody! Glad to be part of the indiehackers community.

I'm working on a project (https://guitarllama.com), which is a guitar music theory course with interactive tools and examples. I've started collecting some subscribers for an email list and only have a few takers at the moment (~40 people). The only marketing I've been doing is a drip campaign through reddit, releasing 1-2 lessons every week or so.

However, I can't get anybody to give me feedback on the site. I have users subscribing to my email list so I can only assume there are at least some users interested... But no users are emailing me feedback when I've asked, or leaving feedback on my reddit posts.

I've also tried to incentivize my email list readers to fill out a google survey I've made by telling them they'll have a chance at winning some yet-to-be-determined prize as a thank you if they do, but nobody's filled it out yet. I add the google survey and ask them to do it at the bottom of every email I send out.

Does anybody have any suggestions on what I can do to get real user feedback on the content? I don't mean feedback on user testing, but on if my content is too dense, too complicated, what other topics they'd like covered, etc. Thanks, excited to hear your suggestions!