Unfortunately low power ddr4 (lpddr4) does not come on SODIMs, soldered ram is the only way to get it. I know everyone likes to shit on apple about this, (and they definitely should about soldered storage) but there are practical engineering reasons for soldered ram. Note that the framework laptop has abysmal battery life relative to the competition.
In-package RAM vs soldered on the motherboard next to the SoC package makes no difference to the bandwidth: GPUs take the latter approach to achieve equal or higher bandwidth. In either case, having a memory bus wider than 128 bits is a major factor in offering higher bandwidth than mainstream platforms. (Wide memory buses are easier to lay out on a PCB when routing to BGA memory packages than [SO]DIMM slots.)
