Thanks for starting the conversation on this, @yuu1412! This one is going to take some thought, but I like the specific use cases you all have mentioned, and I could absolutely see this fitting in with the platform.
So far, I think the best way to go about this would be something like the “keys” approach you mention, instead of passwords on each post. This would nicely support the monetization features we’ve started adding, as well as using ko-fi or simply giving people access to posts automatically like in @anx’s use case. I think as our monetization features keep evolving, we can consider including this functionality there.