Back with more information! So it looks like it’s tied to both scalability and preview rendering level. Medium or below scalability cuts it off if this Cvar is below 4 r.SceneColorFormat thanks to Kaidoom, Ess, and everyone else in this thread below!
So first try to change the scalability to epic, then go over to your RenderTexture and look at just it’s alpha channel and see if it worked out.
If it did, head back to the material, cut out the chromakey (which you WILL need if that Cvar I mentioned before goes below 4) then connect the opacity to a “OneMinus” node (just negates it) then hook it up to your opacity or opacity mask. (I swapped mine to translucent material because it’d be cleaner).
This should be cleaner than the chroma, however you’ll still need resolution to make it nice, which gets expensive fast.