I don't think the Inquisitors are some hot sauce, aside from, maybe the Grand Inquisitor. It was Vader's task to deal with estabilished masters and knights.
I'd put that the Grand Inquisitor is on a weak master level and the Seventh Sister is on a relatively powerful knight level but not above that. At last from their performances.
Yeah, I agree. If he were Ventress level he'd hunt everyone that survived by himself, with the exception of Ahsoka probably but we know Vader was tasked to hunt masters and dangerous knights.
The implication is that the GI is not Late-CWs Ventress level, as she was powerful enough that Sidious ordered her eliminated when he sensed she became too powerful. GI never reached that level and is likely a step below even S1 Ventress.