I was deeeep in the OneNote eco-system. I forced myself out of it and spent a lot of time finding the best alternative so I can save you some time: The answer is Obsidian.

But to get to even 60% of what OneNote does, you need a fair amount of plugins, which lowers stability somewhat and, having tried the three biggest sync systems, you might as well go straight to paying for their sync. The self-hosted sync options do not work reliably.