I can't tell from your post if this is something you checked or an assumption. My small town library not only has space for 20 people events, it has a dedicated tech space used for it-related education and events.

I'm sure not every town has that kind of facilities, but if you haven't actually checked yet, please do. Libraries are awesome and often overlooked.

Alternative idea: Some cafes that close in the afternoon are happy to open as a meeting space in the evening at not-terrible prices.