ROG Ally X can support UHS-II MicroSD cards, so you'd better get some UHS-II microSD cards: https://www.amazon.com/s?k=UHS-II+microSD&tag=6890-20

Get this: Lexar 128GB Professional GOLD micro SD Card, UHS-II, C10, U3, V60, A1, Full HD, 4K UHD, Up to 280/100 MB/s

I use the SanDisk 1.5TB Ultra microSDXC UHS-I Memory Card. For its size, this micro SD card is a beast. It is fast with data transfer and has an incredible storage capacity. The price is relatively cheap for this type of memory capacity.

I bought the SanDisk Extreme PRO microSDXC UHS-I Memory Card 1 TB for $110 at Amazon. Works great on my ROG Ally X.

I have a Samsung Evo plus of 512gb and it's been very good so far. https://www.amazon.com/SAMSUNG-Adaptor-Expanded-MB-MC512KA-AM/dp/B09PZ9SHTK?tag=7649-20

Delkin Devices 128GB POWER microSDXC UHS-II (V90) is a fast UHS-II V90 card for ROG Ally X.

SD cards are outdated and slow compared to SSDs. For similar prices, upgrading to a faster, larger SSD is a better investment. While SD cards may suit some niche uses like ROMs or indie games, their declining relevance in modern tech suggests embracing newer storage solutions. The ease of SSD upgrades in the Ally makes this transition practical.

Get any UHS-II MicroSD cards.

I use this, works well: https://www.amazon.com/SanDisk-Extreme-microSDXC-Memory-Adapter/dp/B09X7C2GBC?tag=5422-20