Shoot em up bullet hell colliders

Hi,
I am doing a bit of research for a shoot-em-up bullet hell style game and would appreciate some advice on how I should be handling the collisions.

My gameplay takes place entirely on a 2d plane, however, the orientation of this plane changes from time to time, with a little cut scene in between, and everything is rendered in 3d. I am considering using 3d colliders and kinematic rigidbodies for the bullets, and I'll need over 100 on screen at once. These need to collide with the player, or the player bullets with the enemies. I am also considering colliding with inanimate background objects (3d).

Is there an efficient way to take advantage of the fact that all the bullets are essentially 2d? I still might want them to collide with the 3d background colliders. Does that mean everything needs to be 3d colliders, and will this be too expensive?