I am building a studio with the exact same pathway. I am still far too new to Unreal to offer an in-engine opinion however there is another way to do this. I have been concerned with being able to grade footage afterwards in post and my “external” pathway option is this:
Camera feed, with camera tracking, into Ultimatte 12. Pipe the Unreal background into the Ultimatte 12 background feed and compose it that way. Pretty traditional. However I use Hyperdecks to capture the Background, Matte, and Program feed for each camera. I then take the background, matte and original camera footage into Resolve and am able to grade both the background and the key any way I want to. I use the recorded matte and the camera “key” footage and this works perfectly.
Interested in everyone’s thoughts regarding this process. And yes, over the next few months, as I continue to learn Unreal, I will be working on how to being both key and matte into Unreal.