I have used many plain text tools over the decades. In the past year, NotePlan has taken over everything that is not coding, or long form writing.

It was easy to move my markdown notes into it.

Plus NotePlan has replaced most of my calendar and task management apps.

I use it on Mac, iPhone and iPad. Looking forward to the web based version that is progressing nicely.

Syncing across platforms is a bit harder than I would like. Dropbox and GitHub syncing work, but it is best using iCloud. I use ChronoSync running on an always on Mac to sync to locations for Linux and Windows access.