You can just make the questions more location or theme specific. There is no way a bot will not slip up on stuff like that, and it doesn’t need to be 100% fail proof either.
We get a lot of LLM bot applications on our instance, and even if it would get 10x harder, they would be still really easy to spot.
One man’s “unstoppable communities”, are another man’s “spam and NSFW content” 🤷
I think you need to make up your mind what you actually want. Gatekeeping access under the guise of fighting spam seems directly contradictory to the stated goal of your project, and assuming it wouldn’t be flooded with such content when most of the internet consists of such was pretty naive.