>>297537531
>>297532808
Here's a hacky attempt. Use this shader in the custom shader slot, only render the Player and MirrorReflection layers.
https://pastebin.com/KkdBCEWJ

Then, you have to make a custom skybox using the Skybox/Cubemap shader and just make it pure #00ff00 green. The shader will make anything close to pure green transparent. It uses basic rgb distance so it's not a great chromakey, but maybe good enough for your use case. To make the scene not actually be pure green, you can put a custom skybox sphere around the world, and add a reflection probe and lightprobes so objects in the scene aren't reflecting green as well.