IMO, new sessions to the sub-board sounds reasonable. Perhaps either the current one should be stickied, or old ones closed (leaning towards the former, as people may still want to discuss things that happened in them, especially shortly after the session), to differentiate the current ones.
If desirable, I'm fairly sure we can configure that board to allow anyone to sticky / unsticky their own topics, and 100% sure we can allow anyone to sticky / unsticky any topic as a fallback if need be.