Basic question: I want to create an action to save a note to a specific (and pre-defined) folder on the filesystem. But not to Dropbox. And not to one of the prescribed iCloud locations Drafts already supports with its default Save to Files action. Is this possible, or is Drafts effectively sandboxed?
If you are talking about iOS/iPadOS, then it is sandboxed, though you can, in many cases, use Shortcuts combined with Toolbox Pro/Scriptable and the file/Folder bookmarks they offer to get around that for something specific.
On Mac, you should be able to at worst output via a shell script or AppleScript.
I do this using Scriptable. Works nicely.