I am using Standard Notes currently but I don’t want to pay 10$/month anymore.
Edit: Guys thank you all for your suggestions. Unfortunately I couldn’t find a one-size-fits-all approach like Google Keep or Apple Notes. I realized that we really dont have much choice as an open source community.
I decided to close my Standard Notes account and started using Gnome Text Editor (it does more than you think) with Syncthing. I am using Markor on my Android end. But it is not really a reliable solution since Syncthing consumes more battery than your average notes app and its GTK client is cumbersome. So even though my quest will continue, I am done for now.
There’s a whole subculture of people obsessed with Emacs’ org-mode for notekeeping, todos, agendas, etc. I don’t really know the half of it, but what I appreciate is the outline view, inline LaTeX and code snippets, nice tables, and export to HTML. Then I use Syncthing.