I miss a general tag which would be a subset of object insertion and a superset of dildo harness. With object_insertion, it's not possible to know whether there's some kind of thrust, maintaining some kind of pressure on the inserted object.
A possible description for vaginal_held_object_insertion would be:
This tag specifies for a vaginal_object_insertion, that one or more of the inserted object are pushed in or held in, by hand, by the ground, a wall, a piece of clothe, a dildo_harness or a contraption, etc.
This tag implicates vaginal_object_insertion (learn more).
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The names I propose may not be the best.
I do not know whether there should be a held_object_insertion tag. Looking for *nal_held_object_insertion might be enought.