While it does contain a lot of FOSS, SteamOS3 isn't publicly available yet so I don't think it counts. You could just put a different OS on the Deck though!
Maybe it's not on sale everywhere, but it's the same price as a 256GB Pixel Tablet for twice the RAM, twice the storage, and full Linux instead of Android.
Tablets are traditionally a closed design, with the OS tightly locked down. Even if that price were high, which I'd challenge it's not aside from the CPU choice (probably by necessity), there's bound to be a bit of a premium for early devices to a niche market.
Starlabs look cool. Their own hardware, coreboot, Linux out of the box, UK company. Planning to get one of their laptops when I'm done with my current one.
Are physical keyboards a must? What about something like a Fairphone running Ubuntu Touch?
Or, for that matter, what is driving the need for Linux? Could it be satiated with an android device and/or x2go to a self- or cloud-hosted Linux desktop VM/VPS
If you mean for gaming, I'm mostly happy with my Anbernic 351M running modified Ubuntu, ArkOS.
If you want a phone factor, the Pinephone is fun to mess with and has a bunch of different distros you can run.
If you're thinking about drawing, see if you can get your hands on a convertible laptop with a Wacom digitizer built into the screen, from Fujitsu. Install any distro.