Assuming they're a nonjump race, it would take time to establish a colony anywhere. Even with continual acceleration, we're likely talking decades per parsec.
The lowerbound is based on your guesses as to distance traveled to Yorbund and the likely reasonable time to travel.
Blah, blah. On to my numbers.
My first thought is that Savria isn't far enough away -- it's only 160 parsecs. I'm doubling it, to 320 parsecs.
Next, I let the tail wag the dog, and claim the exodus from Ssrar to Yorbund took 1,000 years.
Finally, continuous acceleration "compresses" the longer distances enough to make long journeys almost reasonable. Call it an ion engine, call it a ramscoop, whatever.
320 pc in 1,000 years is about 0.004G continual acceleration. It seems reasonable to assume that they made their journey in a series of smaller hops with stronger drives (say, 0.02G).
It stands to reason that any worlds in the same subsector as Ssrar with nasty corrosive atmospheres friendly to the Hhkar would be colonized in (relatively) short order. 25 years to reach one parsec.