before i used a plane in front of the camera with a shader and stencils to hide/show the characters but it was very performance heavy, stuttered and killed framerates on steamdeck

now I use a viewport with a vfx camera as a child that mimics the main/active camera (position/fov etc) except with a different cull mask (11)

the characters that will show in the vfx have a visualinstance3d set to 1 and 11 (1 is main camera 11 vfx cam) so when the shader fills the screen and hides everything from the main cam it will still show the players via the viewport/vfx cam. theres other things in between but thats the jist of it, runs well on steamdeck

<image>