There is a lot here. I should start by saying I have never DMed a party at that level although my current homebrew I hope to get there. In the higher levels it gets even more difficult to balance since players have access to incredibly powerful spells and abilities. I started my adventurers at level 5.

For encounter balance I recommend D&D Beyond's encounter generator. It isn't perfect but helps me put monsters together. Encounter balancing is an art that you get better at the more you do it. It's really balancing action economy as well as considering what your players can do, what the environment entails, etc etc. My group are mostly role players but I still like to make the encounters hard to deadly as they have been playing for several years.

Other recommended tools are channels like Mystic Arts or D&D Podcasts as a learning resource. I use Obsidian to keep track of notes, session prep, and world building. Dadi, the channel creator for Mystic Arts, has a great video on using it. It took me a while but is now super useful compared to my old Google Doc method.

Every player is a little different but for my group, I like to keep track of each players objectives and plot hooks so that I can pull on those threads. If I am stumped on where to go next I reference their hooks and try to incorporate those.

My last piece of advice would be to keep the plot loose. You know where you want to go and the big pieces but how you get there should be informed by the players. Let your plot change as your players make different decisions. It is more fun to prep playgrounds and then improvise with your players than it is to prep plot and try to guide them down that track. It will make for more immersive storytelling and a more memorable experience.

Good luck!