The limitation of Shortcuts to write in files anywhere outside of Shortcut’s own iCloud folder is what made me get 1Writer. Since I only append to my daily note (which also as a side effect opens it) and my daily note is the first thing I create on my computer in the morning, so it always exists, I didn’t think of the case where it is absent.
I just tried using the same append action in the url-scheme but this time tried for a note that doesn’t exist (tomorrow’s date) and it ended up creating a new note for me. So perhaps, using append instead of create is the solution @ryanjamurphy!