Any M2. SSDs will works, just choose any brand you love. I personally love the WD Black SN850X, very stable.

Samsung 990 PRO

Regarding what #5 said about using the 990 EVO, those are decent but honestly, for a handheld that runs as hot as the Ally X, I prefer drives that focus more on power efficiency and thermals. I've tried many over the years and keep coming back to the Sabrent Rocket 4 Plus 2TB M.2 2230 NVMe Gen4. It is specifically designed for the smaller form factor, though it fits the Ally X bay perfectly. In my experience, it runs noticeably cooler under load compared to the standard desktop-sized drives. If you want something rock solid that isnt gonna cause heat soak issues inside that tight chassis, grab that one. Just make sure you handle the internal bracket carefully when swapping the drives out, seen too many people strip those tiny screws.

Any M.2 SSD works, just get best deals.

I bought WD_Black SN770 2TB, love it. https://www.amazon.com/dp/B09QV5KJHV?------123456890?7649-20

I just upgrade my ROG ALLY X with the WD Black SN850X. Works well no problems yet.

I'm using the Samsung 990 EVO in my Ally X. Zero issues, good temps, and decent pricing.