OP was asking for real-time collaboration in a package similar to Google Keep: a simple, mobile-friendly UI (my bar for this is at minimum a UI that has a dedicated button to make a checkbox, automatically adding a checkbox on the next line when hitting “enter”, and the ability to check or uncheck boxes by touching them alone) with an at-a-glance view of available notes, both private and shared.
It’s something that I want, too. I’m happy using tons of weird stuff, but I need something simple, easy to use, and with real-time collaboration to use with my partner, who is very much not interested in anything less convenient than Google Keep. The closest thing I can see coming is HedgeDoc 2.0, but it would still be a hard sell.