Associated with the keyboard of your choice.
But also very easy to DIY by plugging an old laptop or a raspberry pi to an e-ink display/monitor and set it up to automatically start a text editor at boot time. Even a raspberry pi zero would handle that task superbly. I am not sure what prevents you to do that except lazyness if this is really something you want.
The problem with a general use machine is you can always alt tab out and procrastinate. If you're the one who set it up, you're the one who can un set it up.
The keyboard is great, NewtonWorks is a surprisingly powerful office suite and it syncs nicely with the Mac through a serial-to-usb adapter.
It also had BASIC, which I used to play with – and at one later point, I even wrote a little program in Visual Basic to simplify the process of transferring text from the NEC to his PC, including getting rid of the many erroneous carriage returns that the process otherwise inserted.
[0] https://www.old-computers.com/museum/computer.asp?st=1&c=334
I don't write on paper because, as someone who's been reading and writing for half a century now, I learned to write for a living on devices where I could edit the text as I go.
And it's so much better than pen and paper, where every mistake is permanent.
I don't want to hand write on my computer. Draw, yes, but not scribble. I want a keyboard and editing: cursor keys, delete in 2 directions, copy and paste.
Freewrite does not do that. It's a typewriter, but digital.
No cursor keys should have been a giveaway. Later hardware revisions can run a later ROM which adds cursor control using alpha keys, in a way that by the sound of it reminds me of WordStar or Vi in the 1970s.
Or, pick up any Android based e-ink ereader and plug in a keyboard.