The bundle question is interesting.
The model right now is really an unbundling. The direct relationship between writers and readers is what makes Substack work: as a writer, your incentive is to earn and keep the trust of the audience who deeply values your work. That's not just a good way to get paid for work you're already doing. It's a model that allows and rewards a fundamentally different and better kind of work that the work you would have to do if you were e.g. trying to please something like the Spotify algorithm.
That said, bundle economics are real. And so while we wouldn't and couldn't do some top down bundle, if there were a way to do bundling that maintained the direct connection, and put writers and readers in charge (e.g. writer self federation, or readers buying several subscriptions at once) that could be very interesting in the future.