BG3 is a top down CRPG. Having 3D assets and being a 3D game with full 3D movement aren’t the same thing.
And whether it’s more content is debatable. There’s more pure story and production, with a lot of branching, but the overall amount of space (not counting Starfield’s use of negative space because of the setting) is significantly smaller. And even in terms of total number of quest lines, Starfield has a lot. Which you can get more time out of is all about personal preference. There will be people with 1000 hours in both, easy.
You can literally play BG3 as a third person turn based action game, with an over the shoulder camera. It’s entirely 3D.
Turn based and action are mutually exclusive. It is not and does not resemble an action game.
The assets are 3D. You do not play in 3D. You do not cast a spell and have the physics of your interaction calculated in real time while 10 other characters are simultaneously acting and having their spells calculated based on the real time movements of all the other characters. You do not hit a jump button and have where you land determined by your speed and direction. The actual gameplay mechanics are all pure dice roll. There are no 3D physics in play.