I have used StandardNotes until recently. Unfortunately I had to change because I want to collaborate and share notes with my SO. If SN gets that I will probably move back as I already am using other proton services and I prefered using SN. I will continue to pay attention to your progress going forward!
The reason they don’t support Monero was adressed in a recent podcast “OptOut” with the CEO of Proton.
TLDR; they fear the biggest companies for auditing their finances, won’t work with them if they do. Which is reasonable, they are a business after all.
Joplin, not as good as SN or Notesnook, but has collab