More my own theorizing but how much of Hydaelyn was actually a Primal and how much was just
Venat's own natural Ancient transformation ability (think Emet and his 'Hades' transformation at the end of ShB) is a very good question. Personally I think it was a combination of both (Hydaelyn's form taking on Venat's perfered transformation appearence), but provided with considerably more aether that empowered her as a Primal (and allowed the Sundering to even occur).
It would go a lot of the way to explaining why Hydaelyn did not temper, and not just by design - she had no reason to (Zodiark's tempering was something of a mistake, and later Primals had the tempering effect added in deliberately by the Ascians as "extra code" if you will.
But that's just my own theory so please by all means, tear it apart!