Firebrand or Luminary for solo Guardian?

There may not be one clear winner without knowing what “complete” means to you. Try both in the same kinds of solo situations and compare how comfortable they feel, how well they handle groups, and whether you enjoy playing them for a while. Since Willbender hasn’t clicked, judge Firebrand and Luminary on their own rather than against it.

I’d start by picking your main priority. If you mostly want a specialization that feels comfortable to play alone, focus on how each feels in your usual open-world activities. If you mean “complete” as in covering several kinds of situations, make a short list of what you want it to handle and compare both against that list.