My own field of research is a number of centuries after old Geoff was active, but I've always enjoyed the Historia as literature, and have also always been curious about the origins of some of the things in there. Some of it appears to be based on things that happened, though even my Early Modernist self can tell that he gets a lot wrong, but some of it I'm thinking has to have been based on old myths and legends, and I want to know more about those (and about what he may have just plain made up) as well as about any possible real history that he might have also drawn from. I'm especially interested in a) whatever sources he may have been working from regarding the Arthurian legends and b) whatever actual people/events may have inspired him.
King Arthur: The Man Who Conquered Europe by Caleb Howells contains two chapters which analyse books Four and Five of the Historia Regum Britanniae. Those are the two books of the Historia which cover the Roman era of Britain.
Howells doesn’t analyse the pre-Roman or post-Roman eras because sources for those two eras are severely limited, meaning that any analysis would have to be highly speculative and thus not very helpful to determining the general boundaries of accuracy of the HRB (which is the purpose of those two chapters of Howells’ book).
Those two chapters present a detailed analysis of each major event in the Roman era, looking at how Geoffrey’s account differs from the most reliable historical information, and then at the end of each section he presents a list of all the errors in Geoffrey’s version of that event.
Like I said, it doesn’t look at the pre- or post-Roman eras, but you will likely find the analysis of the Roman era to be helpful and insightful.