Linux

I would have said Doom Emacs but given your note about vim, I'm assuming it wouldn't be a great fit for you. Still, I used to write in Darkroom on Windows, because I really liked the totally minimal and simplistic nature of it, and Doom Emacs with writeroom-mode is a perfect upgrade.

As for other alternatives, it all depends on your own taste. I don't think the issue here is really the amount of apps, just finding the right one. You mentioned Writer sends you on a constant formatting spree, so maybe a text editor would fit you better than a word processor like Writer.

In that case, I suggest you look at something that would resemble notepad. Lite XL is my favourite notepad-like text editor but I don't think it's usually available as a package. You can also try Gnome Editor as it is essentially Gnome's answer to the lack of a super-minimalist app like MS notepad on linux. People have mentioned Obsidian and while it's nice, if you're not going to be using Obsidian's graph or linking features I'd say you're better off with a simpler markdown editor, Marktext is pretty nice imo. Sublime text is another good option for customizability, ease of use, and minimalism (Although not FOSS if that matters to you, neither is Obsidian for that matter).

You can also try and find a port of the original darkroom, as far as minimalism goes it really gets it right.

Overall, from what I can gather from your post, I suggest you use Marktext or LiteXL, if possible. Try out one of the other mentioned apps if those don't fit your workflow.

Edit: For clarification, these are my suggestions for writing, formatting is a completely different practice and might need other tools.