There is also a usability issue in desktop that makes finding the tagged cards rather effortful.
Could you elaborate on this? Why is it effortful? What would make it easier?
I don’t think there’s a way to find notes created on mobile. However I do something similar to you, in that notes that I create on mobile are typically stubs. For that I have an extra field, let’s call it “Issues”, which is not used on the Card template, and is also sticky. I just type in “x” into this field, and then in AnkiDesktop I can search for Issues:x. It’s not much better than using tags, admittedly.
Maybe this could be a future feature of AnkiDroid and AnkiMobile: automatically assign a private tag depending on which platform a note was created on
If the goal is to mark notes that need further editing, you’d presumably need to remove such tags afterwards, so I’m not sure that would save much time. Why not just write “todo” in one or more fields you intend to edit, then search for “todo” later?