Thank you for this, @JFallows.
However, as the title of this thread may tell, my focus here lies exclusively on Exporting (many) notes as separate txt-files; not on copying and then pasting them manually into externally created txt-files.
And: I don't want to export all notes into one single txt-file either.
I do want to export all notes into separate txt-files - as done in @galen's kindly provided "plain-text-export.tbx". However, this involves some steps of creating a Template and setting .html to .txt (with or without a Stamp) and so forth. So I could create a new Tinderbox file, start brainstorming and jotting down ideas and then quickly exporting those notes as separate txt-files to my nvALT-Folder or into Devonthink.
So (since there is already an option in Tinderbox of "Export ... as Text") I was just wondering whether it would be possible to add a(n additional) checkbox in this very "Export ... as Text"-Dialog that would additionally offer the possibility of exporting all the notes as separate txt-files beside the default-option of only exporting all the notes into one single txt-file.
For a non-programmer like me it's rather counter-intuitive having to create a plainText-Template, using an HTML-Export-Routine, renaming .html to .txt in order to simply export notes as txt-files; especially once there is an "Export ... as Text"-Option already build into Tinderbox.
But again: Galen's suggestion works well.
Thank you all for being so pc (= passionately cooperative)!