I'm curious how something like CUDIMM memory would perform under the same workloads.

Currently avoid machines with soldered memory, but if memory can be replaced and still have similar performance, that would change things.

PCI lanes just aren’t there yet which is why the soldered memory approach to shorten the gap and increase the speed. The memory the framework desktop uses doesn’t come in slotted form (LPDDR5X).

You absolutely can (and should) build your own for slightly cheaper. Just find the fastest DDR5 CUDIMMs you can paired with the fastest memory bus mobo.