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 