If you wanted to share some medium sized ideas with this group specifically, where would you put them? I could write them in my blog, but for FoC would sound like "subtweeting" and for outsiders it would sound out of context (because I would assume a lot of shared context of things that here are shared but outside they are not)

I think the blog sounds like the best bet. Linking to the blog post with a paragraph length summary written specifically for FoC as the Slack post seems like a reasonably compromise?

Google Docs. Especially if the point is feedback, because it has inline comments that stay in context even if you revise or add to the text.

Those feedback features are good for things like proofreading, but I don't think they'd sustain a deep discussion.

Yeah, but there’s a huge gap between “proofreading” and “deep discussion” where inline comments are great, in particular because they let people point at what they’re talking about, and because it allows other people to stumble upon discussions in context. (Speaking as someone who has discussed medium-sized ideas in Docs as a Google employee for five years.) At any rate, using a Doc doesn’t make Slack disappear. 😅