Posting on Indie Hackers has worked best for me so far. Both positive and negative reactions are useful. That said, it's inconsistent - some posts get real discussion, others get nothing. DMs are hard. X and LinkedIn both have weak response rates. The exception on X is when you catch someone tweeting about the exact problem at the right moment - then the conversation opens up naturally. The surprise was the Stripe App review process. The back-and-forth with the reviewer surfaced gaps in my feature set I hadn't noticed. The application process basically became a product review.