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The on package DRAM for the current macs isn't anything special. It's just normal LPDDR running at normal LPDDDR speeds, same thing you find in your phone. On package has many advantages, but it's not a hard requirement.

The bigger is how much LPDDR5 memory they can hook up to their 512bit memory controller.

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Maybe the very ability to make a 512 bit-wide memory controller stems from the very short, very tightly controlled and shielded lined between the DRAM chips and the controller?

(No idea if it's so, but DRAM is such an analog HF beast that a controller needs training before it can work with a particular chip, in a particular slot on a particular system board. It's automatic but it's there, and it must have limits.)

High end graphics cards have been using 512 Bit wide memory buses for a while, so I don't think that's it.

but the GPU RAM is not expandable and seems to be a big price factor.

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