Go to your audience.
If your audience tend to reside inside of niche FB groups, join those groups, perhaps advertise in those groups. If your audience is geographically located in clusters, target them based on their location(s). If your audience watches a lot of YT ....
The critical path for business success depends on ones ability to know where their customers exist.
You could end up entering entirely new industries just to get your newsletter to succeed. I like to create multiple platforms for one objective. For example, if I wanted to market a newsletter, I would go to my audience, and I would consider establishing a website, a blog that lightly touches on the content from the newsletter, a YT channel even could be useful where you create video content from the content in your newsletter ... assuming that is legal. And/Or maybe start social media accounts that highlight the content in your newsletter. There are a lot of examples out there of social media accounts and websites that are just part of the funnel to the newsletter or similar product.
Small business owners are expected to know much more than their specialized skill set.