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UPDATE: I couldn't get AutoInput to select text in SMS. I remembered that Jota Text Editor comes with voice recognition that works offline, and AutoInput friendly menus for select all and cut.
I bind the select and cut task to a swipe, and have a profile to watch the cupboard and run my AutoVoice task.
I use a couple of tools. The first is Workflowy (workflowy.com) It's useful for keeping track of character names, locations, and that kind of thing. (Aka, the "what did I call that diner in chapter 2?" problem.)
Then I give everything a pass through Grammarly (grammarly.com) It isn't perfect, but it does a good check on spelling, punctuation, basic grammar and that sort of thing.
As for actual writing, I use a dumb text editor. Either Notepad++ (https://notepad-plus-plus.org/) on my main machine or Jota on my Chromebook (https://play.google.com/store/apps/details?id=jp.sblo.pandora.jota&hl=en_US&gl=US)