Honestly, looking at how much customization is going into the "Dragonriding mounts"... I can't but feel like, THOSE should have been the "Dracthyr". Scaled-down in size, obviously, but that honestly just seems SO MUCH MORE exciting.
Furthermore, there would be absolutely zero question about "why can only Dracthyr be Evokers", because they'd be actual full-blown (albeit smaller) dragons.
And then just open up with "Visage" form to any race, since you would only really use that form for non-combat functions.