I find Room Relations hard to work with, not very flexible and much of the work is done under the hood in a way that is hard to really be sure how.
In my projects, most of the time I just create presentation objects – objects dedicated for some UI presentation that can be filled with a custom select.
That way I have much more control over what I want to fetch from DB (i.e. what I really need), and I fill that into that custom presentation object.