That was my starting point, but I figured most people would want to naturally segregate this stuff by their follow list anyway in the beginning. Now you can argue that a 10xxx list won't be great either because I may want to have separate lists for different types of content, so maybe use a 30xxx list and give different relays different list ids to use? You could also use a proprietary database that is configured only in that specific relay and doesn't sync anywhere and that wouldn't be bad either. There are many ways of approaching this.

We could also use individual events since I think these lists are going to be massive.

That seems like a hassle to maintain, no?

Another client can simply automate it by looking at the paid list for the month and regenerating all events.