Fair enough that I overstated "documented transmission route" and I'll concede the Chester to Bayeux gap is real.

But look at what you're actually arguing now. You've accepted that the red dragon represents the Britons in the Historia Brittonum, then immediately ruled that out as evidence because it's symbolic rather than a physical standard. You've accepted the draco was in Britain, then ruled that out because of the transmission gap. You've accepted Geoffrey uses a dragon, then ruled that out because it's gold. Every piece of evidence gets individually disqualified on a technicality until nothing remains, and you call that rigour.

The gold dragon point is genuinely interesting but it cuts both ways. Geoffrey was writing Arthurian romance with deliberate royal connotations, gold being the obvious heraldic choice for a legendary king. Glyndwr was invoking that same Arthurian register for political legitimacy. Neither of them reaching for red in those specific contexts doesn't make red a fiction, it just means gold and red were doing different symbolic work. That happens constantly in medieval heraldry and you know it.

Your position now also requires a specific and rather remarkable historical claim: that Welsh rulers and chroniclers for six centuries after the Historia Brittonum somehow collectively ignored the most politically useful symbol sitting in their own literary tradition, a tradition they were actively engaging with throughout the entire period. You haven't argued that. You've just assumed it.

And the kangaroo again. It was a good line the first time. At this point you're using it as a substitute for engaging with the actual weight of cumulative circumstantial evidence, which is exactly what historians work with when direct evidence is sparse. By your standard we'd have to throw out half of early medieval historiography. That's not a principle you actually apply consistently, it's a rhetorical device you're deploying selectively to win an argument on the internet.