I think you might want to take a look at novelWriter if you want a writing app. However, that one uses a markdown-like syntax, and it seems you want either just plain text or rich text.
For that, I use Obsidian with a few plugins and a theme that allows me to focus, that being Zen. Otherwise, that uses markdown, and that might not be a good option for you either.
Doing some Reddit research (I'm reading using Eddrit, though), it seems FreeDOS with something like WordPerfect 5.1 might be a good option, but it isn't Linux. You want Linux, so that isn't viable either.
Someone seems to have recommended Tiny Core and whatever editor (TUI editor) you'd like. For that, I would personally use Vim set up as a word processor, as shown here in one method, but that seems to be something interesting too. The only difference is that I'd want to see if Harper is a package so I could use that instead of LanguageTool.
Wordgrinder is another TUI option, which is a terminal-based word processor, though I hadn't really used it.
Kate is another option, which I've used quite some time back. I don't really care for it, but it could be something more your style.
That's just how I see it, though.