Hi,
It is actually possible to create a UI template. For this you have to follow the same steps as for creating an application template as explained in the corresponding article. When you will edit the JSON file, you will need to edit the TAG section and write "TGUI" instead of "TGAT" in order to be able to see it in Designer in the "UI template" section.
I have discovered an issue recently with creating application templates with the latest TouchGFX versions. You will need to use TouchGFX Environment 4.14 (or 4.13) instead until this is fixed.
Otherwise the solution Alexandre gave is very easy and quicker to do as well :grinning_face_with_sweat:
/Romain