It's the 128gb variant, and I chose it for that reason exactly. I can dedicate up to 96gb to the iGPU

You can probably just dedicate 1GB for the framebuffer, and then let Linux dynamically allocate memory to it at runtime. As far as I can tell this doesn't impact performance, so there's no downside. (Older AMD stacks required a static partitioning under Linux, I think).