Yes, the live-action film series (commonly know among the Transformers community as... the live-action film series) should at the very least have its own tag.
It's typical for us to do so. See marvel-cinematic-universe or nolan-batman-trilogy.
We've also applied tags for individual movies, in the case of star-trek-2009, star-trek-into-darkness, and star-trek-beyond, even though they all also belong under the franchise tag of star-trek-reboot (although it's not applied correctly to all matching questions, that's a different issue).
In this case, I don't currently see a need for individual film tags.
I would recommend transformers-live-action or transformers-live-action-series with a wiki excerpt that specifies it's about the live action films as well as the related tie-in media.
The number of questions is relatively small, so that shouldn't be seen as a hurdle in fixing this (which is the case for harry-potter, which is, as usual, grandfathered in as an exception to normal tagging practices).
I've accepted this answer and have started adding transformers-live-action-series to relevant questions.