I think 136-209 is the smartest move for F&BII. It allows him to cover 75 more years while still not spoiling any D&E stories. Also, while the page to time ratio could be lopsided in the first volume (Aegon I's reign after the First Dornish War is covered in a few pages; the first dozen years of Jaehaerys' reign vs. the rest) Martin was pretty good at having the pace match what was narratively intersting. But when you think about how much gets teed up at the end of F&BI plus the things we know Martin could write a lot about over the next 75 years (the Conquest of Dorne, the reign of the Unworthy, the First Blackfyre Rebellion, all the crazy shit going on with the Starks in the second half of the second century), it seems pretty clear that there's no way he could do this all in one more volume. Unless of course he skimps on the 209-283 period in favor of D&E stories, but that kind of defeats the in-world premise of it being a complete history of the Targaryens.
He has kind of made an interesting challenge for himself, telling the same story via multiple perspectives and series of books. I'm curious to see what his solution is.