2. Make a blog post and share on HN
3. Let the commenters tell you everything you did wrong and what they would have done instead
Put more seriously—I think if you don't have an external professional network to lean on, then you should blog it, talk at conferences, etc. Saying "we aren't sure this is right but it's what we came up with" will definitely invite comment. Keep an open mind and remember any publicity is good publicity: your harshest critics will still be engaging and giving you feedback.
I don't think chat services (IRC, Matrix, Discord) are a good fit here, unless used to talk to a group of people you already know and have some cachet with. But I might just be old-school.