Regarding Joplin: I don’t know what you mean by callouts, but it does have a plug-in system. Perhaps there is one containing what you need?

If not, and it’s not beyond your skill set, you could build it yourself.

That looks comfy, and I like the cutout for the coccyx.

Do you need something specialized?

The biggest roadblock will be whether the bootloader will allow another OS. You should be able to search xda forums for your device as a start.

Linux can be installed on ARM, no problem.