I would love to see a real in-depth documentary of that post-Walt leading into Orlando period, but all I can think of are more recent films that are usually piecemeal and/or much more about what’s in the park, rather than the brainstorming and design that created them. Older ones that Disney Co. themselves created sometimes show promise, then sadly quickly devolve into pseudo-marketing obviousness.
A few books I’ve read have touched on various parts, but lately I feel like I have to rehash all of the Walt background to find tiny new tidbits. (Luckily, I love reading about Walt, go figure.)
Here’s a list of films someone posted online I’ve planned to try to dig up over time.
In general, you might want to check out Great Big Beautiful Tomorrow if you haven’t seen it.
The Defunctland YouTube channel has some interesting stuff, but you have to wade through a lot of other things. Also some cool videos on Martinsvids and I like this Modern Marvels episode.
For EPCOT, the book Building a Better Mouse was interesting, although written mostly by an engineer it sure reads like it and is far from comprehensive - more like some crazy stories of putting it together than a real thread of the design path.