If the ease of repairs influences your choice in handheld gaming PCs, you might want to consider the ROG Ally. iFixit just finished a teardown of ASUS' machine, and it's noticeably easier to open than Valve's Steam Deck. For one, the battery is much less painful to remove — it mostly involves removing screws where Valve's power pack is glued in. It's also relatively trivial to remove the thumbsticks and their circuit boards on the Ally, so you might have a solution if stick drift becomes a problem. As on the Steam Deck, SSD upgrades are simple if you find a sufficiently tiny drive.
The only truly repair-hostile component is the display, iFixit says. There's a lot of adhesive that could make removal difficult without the right tools. That's not a major issue if the entire screen breaks, but may be a hassle if you're trying to put a screen in a new cover.
There's one catch: ASUS hasn't committed to offering replacement parts for the ROG Ally. We've asked the company for comment. For now, at least, you'll have to source parts from other users willing to give up working components from otherwise broken devices. iFixit currently sells some official Steam Deck parts, such as screens, button assemblies and storage.
While teardowns indicate that the Steam Deck is easier to maintain than you'd expect, Valve discouraged repairs early on. It was concerned that merely opening the system created risks, and that third-party components could cause problems. ASUS isn't inviting do-it-yourself repairs as of this writing, but the modularity may be useful when right to repair policies are becoming more commonplace.