Ok, so help me connect the dots, I don't use LLMs, so I'm speaking from a position of ignorance.
For the sake of argument, lets say the thread ripper uses system memory for both GPU computations and cpu computations, its running at DDR5 speeds. Wouldn't that be slower then a threadripper using GDDR7 for LLM processing and the system ram for non-LLM tasks (I'm assuming the GPU is a 5090)?
Apple silicon has memory on the Soc, so its an apple/oranges comparison. The memory bandwidth on the M3 Ultra is 819 Gbps, and up to 546 Gbs for the M4 Max. This is where Apple's architecture shines, so if you want unified memory, or high memory bandwidth Apple Silicon is really your only option.
One of the biggest gripes of intel processors with iGPUs for years was the fact it relied on system memory that was significantly slower then vram. What you're proposing is a return to that slow inefficient approach
Another option is to find a desktop with a Ryzen AI Max+ 395 While it uses unified memory, the max amount for the gpu cores is 96GB, its a close approximation of how apple has its chips