To be fair you can just keep using Obsidian or similar apps on your phone for this, along with an external sync solution like Syncthing.

I tried it for a bit in the past, the main friction is apps formatting Markdown slightly differently (indents, blank lines).

The beauty of an interoperable format like markdown

I do too. The setup I've described is best for big screen work when I need tools that dont compromise in any way.