As you point out, it clashes with fire2, so it can never really be fixed with just a potency buff since one will always be made useless by the other with just this.
That said, fire2 is in the dumpster right now anyway, once you can get umbral hearts you will never use it again.

I do not feel like freeze was meant to be the umbral equivalent of fire2 anyhow, that's what ice2 is.
It's learned almost twenty levels after fire2, unwieldy ground targeted and with a longer than usual cast time.
It's also a BLM spell specifically which is supposed to be a notch above thaumaturge.
It should absolutely dumpster fire2 in usefulness.