LinuxSBC

It would be largely fine, but be careful. Being immutable, a lot of things that you would expect will work differently or not at all. I would not recommend it, but if you're in for a challenge, it's not bad.

When you press a button on this revolutionary machine, it will automatically left click for you!

On GNOME, I like BlackBox, though Prompt looks promising once it's stable.

A combination of heaters and being mostly deployed in warmer environments, I'd assume.

Most of what you said applies to the Linux kernel too. It's good to have other options, but being popular does not mean something is bad.

That should work, though you may want to look into Framework instead.

.ovh domains are like $2/year, if that helps.

Also, KOReader adds a bunch of extra features and functionality.

Kindles are really hard to root. Use XDA Developers forum for this kind of thing. https://xdaforums.com/t/fire-hd-8-2018-only-unbrick-downgrade-unlock-root.3894256/, which is what that guide is based on, looks like it only works for the 2018 version and seems way more difficult and risky than most ROM installations. Also, that won't install Linux, just a different Android version.

Honestly, an actual ereader might work out better for you if that's all you plan to do.

You can't do that. Installing custom ROMs on Android devices is very different than installing an OS on a desktop/laptop. Most devices don't allow changing the OS at all, most of the ones that do don't have any Linux builds, and then you'll be stuck with whatever distro you're given (probably UBPorts or maybe PostmarketOS) rather than choosing your own.

What do you think an API is? They have reverse engineered the iMessage API and are using that to connect to the iMessage servers. It is literally impossible to do as you suggest (use entirely their own resources) because iMessage is centralized and cannot federate with any other server, even if one did exist.