I’ve been using Zim, because I wanted something that was completely brain-dead simple and also completely not in any sort of “cloud.” It’s entirely local to my hard drive. It stores its files as a folder of markdown files too.
How non-cloudy is Obsidian? I might take a look at that.
It’s completely local unless you specifically opt into cloud options. There is Obsidian Sync but that’s completely optional, and your files are still on the computer. I know some people make their vaults Google Drive folders, which, again, is something you have to deliberately do.