You don't have to do everything in a single playthrough. You probably shouldn't try to do everything in a single playthrough. They added so much content because they want the game to be replayable. If you're a casual gamer, then play casually and don't worry about not discovering everything. Larian advises against save scumming and going back to a previous reload because you don't like the outcome of a given choice. I'd recommend just playing organically and rolling with your choices. Leave stuff behind if you can't figure it out. There are only a few sections where you may have to look something up to figure it out and progress forward. None of the optional stuff like the Necromancy of Thay is mandatory. You'll be plenty powerful without it.