Reeves isn't going to design the franchise to where the audience has to watch eight hours of a limited series to understand what's happening in The Batman Part II. Yes, the series bridges the gap, but it's more like bonus content, and basically a visualized/dramatized version of what will almost assuredly be summarized via dialogue or made obvious in The Batman Part II. That, and Batman/Bruce Wayne will likely only make a brief appearance in the final episode.
That said, for those who even dabble in streaming, how is Max not one of your core subscriptions? In terms of their overall library/churning out great, consistently new content, it's easily one of the best streamers out there. Also, closer to the release of the next movie, you could simply spend the price of a single movie outing on one month of Max and knock out all eight episodes in a matter of days/weeks. This isn't some impossible task/big ask.