For your Reddit posts, did you target a subreddit that might have interest in the extension or did you use one meant specifically for extension development/promotion? If the latter, which one?
I'm in the process of adding payments as well and I'm starting things off with Stripe. Specifically, their payment links and subscriptions features. Longer term, I may switch, and am building things in a way that migration will be easy.
Great ideas, thanks. I've had similar success searching for common problems using Google, finding the highest-positioned Reddit result, and leaving a response with a link.