Martin has written a decent amount about Targaryen history in the Fire & Blood book he released a few years back, so much of what should take place on the new HBO show isn’t exactly uncharted territory. It’s not written as completely as the Ice & Fire books, obviously, but it’s also not like the writers are going to have to come up with everything from scratch like the GoT writers did the last couple of seasons.
It’s also possible that they have a better writing room than Game of Thrones did.
If I’m being honest, though, I think a big part of why the last few seasons of Game of Thrones fell apart was because the show runners lost their passion for making it. They clearly wanted to move on and so they rushed their way to the finale to get it over with instead of taking their time and working their way to it naturally.