I swapped the SDD of my SteamDeck. This is not a How To, I just wanted to document my experience with system performance after the upgrade.
Short answer: It is a must have. DON'T USE THE 64GB SteamDeck, buy the 256GB SteamDeck or upgrade the 64GB SteamDeck IMMEDIATELY after getting it!
Games run the same, no difference. I will still install most games on the SD card. BUT the system performance is crazy fast compared to before. With the 64GB eMMC SSD it felt like an underpowered laptop when using KDE or the SteamDeck interface. With the NVMe SSD, the system feels as fast as this type of hardware should feel. This just confirms my hatred of eMMC. It just kills performance in all devices when it comes to responsiveness. Now it is actually fun to do things in the system and not haveing to wait for the eMMC storage to do its thing. If you can, get the 256GB model of the SteamDeck or buy the 64GB model and upgrade the SSD IMMEDIATELY! Trust me! It's worth it!
The shielding around the original SSD was too small for the new NVMe SSD. I had to cut it open, put it around the SSD, close it with a small piece of duck tape and then place a small piece of clear tape around the hole SSD. The ends of the clear tape are underneath the SSD and are pressed against the mounting surface after screwing the SSD down so they cannot come loose over time.
I don't have any problems with the new SSD or with any WiFi interference. I used a "Sabrent 2230 M.2 NVMe Gen 4 256GB". It was, as already meantchend, a little to big but you can make it fit if you really want to. It was the cheapest at the time. IDK if it is any good. Any NVMe SSD from a reputable brand should work. This hole upgrade only cost me about 50 Euros. Also: Be careful with the screws. Different lengths and use the right bit, they are a very soft metal.