Have a look at Obsidian. It runs on a variety of devices, you can sync either with their system, or pretty much anything else, as it just stores your notes as markdown files, and you can arrange notes like that with the canvas system.
I actually had Obsidian installed and it looks like something I'd like if I spend more time on a computer but nowadays I'm mostly using phone for Internet activity. I couldn't get canvas to work on my phone for some reason. It also lack reminders which is wuite crucial for me.
Fair point about reminders, that's not something I use it for, so I didn't think of it. Canvas seems to be working now, and there are regularly push updates, so one of those might have fixed it.
Honestly, the closest I have found is https://github.com/baggachipz/tinylist It looks like keep, can share and edit files with other people, which is something a lot of things are lacking and I use it extensively for that. Also, I don’t like the recommendations of using MD apps/files for a simple checklist/random notes app. That’s way too much for something this simple and I use Obsidian as well. But they serve much different purposes.
It allows you to host your own database for it, and there is a guide on it.
Hey, that TinyList.app is actually very good but unfortunately it's missing a quite crucial feature - reminders... Nevertheless I will be watching its career with great interest.