Theoretically, this 1TB M.2 2230 SSD should be compatible with the steam deck and might be a great addition to a 64GB model for adventurous modders : r/SteamDeck
Buy the 256GB M.2 NVMe Internal SSD 2230 - with single notch - Brand may vary ( 256GB NVMe 2230 M.2 SN ) online - PBTech.co.nz