> Or even a pay-per-article model - some of the free newsletters I'm on publish paywalled things I'm interested in, but I don't want to subscribe to the full deal. But I'd pre-load my account with $50 and pay a couple of dollars for a single article.

This is an excellent idea! Tracking individual articles for payment, rather than the whole publication/newsletter, would allow for bundling in a way that respects Substack's intentions of preserving/respecting the author and reader relationship.