I feel like a solution similar to m.2 could work, holding the module in place with a screw. I dont think the m.2 connector would suffice as it only has 67 pins and DIMMs have I think 288 pins, so that's quite a difference. I do think SODIMM has less pins, but not much less, definitely not to the tune of less than a quarter as many.

Having recently looked inside my steam deck to upgrade the storage, I honestly think they would have had room for at least one SODIMM slot, but the tradeoff is increased thermals, more power draw, and probably some design constraints around the pcb leads possibly leading to increased overhead or latency. I agree that a new form factor would be best to address these issues. It would be cool if something similar to the SXM socket came out, having a pad of pins so you can increase the amount of pins while taking up the same space.