Nextcloud notes + clients on mobile + qownotes on desktop are pretty good for me.
I think it would be difficult to find a text editor that DOESN'T support some for of formatting, most commonly markdown.
With that said I'm going to suggest Joplin, as it fills all your criteria besides also having markdown.
If you need something even simpler, what about a folder of textfiles with some simple text editor?
I wasn't precise, I meant many apps force some kind of rich-text editor that feels clunky and unnecessary, that's what I want to avoid.
Since I'm already using NextCloud, NextCloud Notes seems like a good fit, I'm wondering how I missed it...
If you want to read it on the NextCloud web instance then maybe NextCloud Notes. Just cannot remember how it worked but I did play with it and I think this is true.
On the other hand if you want end to end encryption or otherwise do not need to read it on Nextcloud then Joplin. I sync my joplin though Nextcloud but I can only read it on my devices.
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