If you look closely, my numbers don't
quite add up -- $50/mo is $600/yr not $500/yr.
I was guesstimating about $200k/yr in operating expenses (and that's with a lot of volunteered expertise, as you note).
But structured as a social-purpose non-profit, with everyone paying in being a member who elects a management board, who allocates the money as they see fit (perhaps with some constraints chosen by members), I think this actually bypasses a large amount of compliance work -- if you never return the money, and are strictly a non-profit investing in local businesses which rolls returns in to future investments -- because the members aren't actually making investments: they're paying for a non-profit's operational funds to support local business ventures. Structuring this way also makes for a better case to get donated expertise, eg from local lawyers and investors, and probably makes your tax situation a lot better.
You probably need some rules about earning lifetime membership after a certain pay-in, and definitely some about conflicts of interest (eg, funding member's ventures) but those are more technical details.
It's not a good financial investment vehicle, in that even in the best case you'd almost certainly have negative returns and in the worst, you'd have no returns at all -- but if your goal is changing the function of the market to support local enterprises and you're willing to pay in to make that happen, I think it'd function excellently. (Of course, you only really need to seed one or two Starbucks to have the whole portfolio right-side up again, even after decades of total losses. So at a generational level, it might work as a financial instrument as well.)
Worth noting, even as a non-profit, there are ways to "return" the money besides the value of the businesses started if you do find yourself with extra funds -- investments in local parks, schools, art programs, etc. It'd never come back in currency returns, but I expect "investors" would see tangible value in their lives, quite possibly exceeding the monetary value they put in.
tl;dr: It's easier if you structure as a one-way investment in your community, and collect dividends from the output of a vibrant community.