no, valve wont do it. any computer repair shop could do it but even you could do it. the ifixit guide make things extremely simple and upgrading the internal memory is simple. you just need 2 tools and to follow simple instructions to do it and the process is fast even the first time.
in terms of ssd… any 2230 nvme will work, either pci-e gen 3 or 4 is the same as the storage slot speed is capped. you can go from the cheapest realiable oem ssd to a well known brand one and they will be really similar in performance while playing. just make sure to check guides on nvmes on youtube so that you dont buy an ssd or something you cant use by accident.
there is no point in buying a deck for emulating things up to ps2. any modern snapdragon phone with 8xx/8 gen x chips has been able to do it with upscaling since almost 2 years at this point.
hell, a handheld with a snapdragon 8 gen 1/2 was recently released and it has way better battery efficiency than the deck while doing the same emulation as the deck for the same price.
you buy a deck for the other things the deck can do.