>UI
I'd say, in no particular order:
Reduce submenus and combine them into one as much as possible
If no reduction is possible, group things together and show that group on the menu (such as: "Dungeon Stuff" contains the start dungeon submenu, all the modifiers, etc)
Hide the things you have no access to (or heavily gray out the buttons)
Don't give player the ability to edit the UI until you're sure they're ready for it
Show don't tell: remove text boxes telling you how to use the UI and instead have the player learn from objectives (this is hard to do)
Example: Instead of clicking a submenu and having a textbox appear and say This is where you do X, have a popup while you're doing something somewhere else telling you "Hey go do this. It's in X menu, it does Y" and leave it at that.
Highlights around a button in a strong obvious color when the button wants to be clicked for progression reasons, or if it unlocked. Think of a blinking red button from the movies - you need to steer the player's movements when your UI is huge.
Your inspiration should be modern releases. They have soulless, but incredibly functional UIs. A player is NEVER getting lost in a modern release's UI, and it's not because they're not complicated. Just make sure to keep the soul!