Her lightning feats are a magnitude higher than Dooku's. Her speed is such that Lord Scourge was actually barely capable of parrying her attacks, and he himself can move as a blur, blindingly fast to droid photoreceptors and move faster than an autotargeting blaster cannon can track.
Dooku tier seems like a very conservative estimate.
Dooku killing Ventress with Lightning whilst mortally wounded is incredibly impressive and actually better than Nyriss' feats. The only one she has that is better is the one that would've turned Meetra and Scourge into ash, but that was with a free charge-up on a potent DS nexus, not to mention that she wasn't wounded in the slightest.
Dooku had just collapsed in pain over a blaster bolt shot. You could argue he was building up power in secret (that'd just be horrible writing with the other two not noticing) but it's still not the same sort of charging up as Nyriss, who was gathering visible, palpable energy, and on a DS nexus. Once again, Nyriss was fresh while Dooku was on the ground in pain from a blaster wound.
The circumstances are immeasurably more favorable for Nyriss, so of course she'll have the better feat. Ignore the circumstances, and Dooku's better, IMO.
Pretty sure Nyriss was just building up the energy from within herself to annihilate Scourge and Meetra. At least, it's not remotely implied she's tapping into the environment in a way that, say, Zannah did with Ambria. And instantly ashing herself/Meetra/Scourge with the FLS is indeed an order of magnitude beyond Dooku, Kek.
What about her dueling skill relative to Dooku, tho?