Well I did it, I managed to stick a 2TB NVMe 2242 drive ($120 used) into my Steam Deck 64GB! With a little bit of modding, pls don't try at home :)
Amazon.com: Samsung 980 PRO 2TB PCIe SSD - 7,000 MB/s 4.0 x 4 M.2 NVMe Gen4 Internal Gaming Solid State Drive with V-NAND Technology for Laptops Desktops and Crypto Chia Mining -
Amazon.com: KingSpec M.2 SATA SSD, 256GB 2242 SATA III 6Gbps Internal M.2 SSD, Ultra-Slim NGFF State Drive for Desktop/Laptop/Notebook (2242, 256GB) : Electronics
New Branded Western Digital/Samsung/SK Hynix 128Gb PCIe NVMe SSD M.2 2242 Drive | eBay