I'm not sure what you mean - I think the mobile 300 series can do quad channel already for its APU at least. I'd assume it can do more but do you not need more slots beyond that?
There are a single-digit number of products using the AMD Strix Halo mobile parts that have a 256-bit memory bus. All other mobile x86 processors (including AMD's mainstream Mobile silicon used for these desktop processors) have the usual 128-bit bus.