Yep… that’s about it. You’d need to make a new comp, with a linked or duplicated camera in it and drop that into your main comp (not collapsed) as an overlay.
It can no longer respond to comp lights in the main comp or embed itself inside the 2.5D space.
You might also consider soloing it and rendering out just that layer with alpha channel with DOF off or with draft 3D settings, then importing the pre-render into your main comp.
Same effect as a above.
If you really wanted to embed it in the scene, you could use expressions to turn layers on or off, based on whether they are nearer to or further away from the active camera than your layer.
Something like this on opacity…
C = thisComp.activeCamera;
L = thisLayer;
L1 = thisComp.layer("embed layer"); // the layer you want to embed
myDist = C.toComp(C.fromWorld(L.toWorld(L.anchorPoint)));
L1Dist = C.toComp(C.fromWorld(L1.toWorld(L1.anchorPoint)));
tOpacity = value*(myDist > L1Dist); // show if further away
tOpacity = value*(myDist <=L1Dist); // show if nearer
Then you could render three separate layers and composite them together afterwards. You could even use a central switch to select whether nearer layers or further away layers are displaying. And you could also do this using live compositions… A spiralling world of insanity awaits you…
My only question to you though is…