I agree there is too much stuff in BG3 when you get past 8th level or so. I have many cool magic items in my inventory, but I always forget they are there. Heck, I even forget to use powers of some of the items that I have equipped!
I think one reason why the high level spells are looking difficult to implement is because a lot of them have been pushed down to lower levels already. Here are just three examples: Revivify (3) is essentially Raise Dead (5). Spirit Weapon (2) is the same as Mordenkainen's Sword (7). Blight (4) is the same as Horrid Wilting (8).
Here's a suggestion to make some high level spells interesting & useful but not overpowered: allow concentration concurrent with other spells which also require concentration i.e., by using a high level spell you can have more than one concentration spell active.
The Wish spell is something I wish to see. BG2 did a decent job by having a menu of options which changed according to the caster's wisdom. The oldest D&D wish description I know of, going way back to the original D&D Monsters & Treasure, is as follows: "Wishes that unfortunate adventures had never happened should be granted." You know what that is? It's loading the last saved game.
