Can you not access the battery in the Steam Deck? I haven't opened mine up before.
I also haven't opened the Switch before either though. Is it not possible to just swap out batteries like you could on the DS and 3DS? I had to swap out my 3DS's battery a couple years ago when it ballooned and always just assumed it was an option on the Switch too if it ever happens there.
Edit: Just took a look at the iFixit video, that's a much more involved process than I was expecting. Hate that they used adhesive on the battery, and didn't even add a pull tab to get it out.