Stuck? More apparently, they choose to be a Delta Quadrant power. Species 8472 was a lot more tasty morsel that ordinary folks. What would have drawn their interest to the AQ thousands of years ago? As we saw when Q introduced the Borg to the Enterprise crew, they were not aware of humans at the time. And afterwards, thought the possible technological gain from humans only justified a fairly minor expenditure of resources.
The Dominion's cloning tech may be more advanced, but they couldn't overcome the virus introduced to them by Section 31, so, there is no question that they are, or would be vulnerable in the future. Even the cloning facilities, not being mobile, are vulnerable.
Clever viruses and genetic engineering have limitations, we have no evidence that these tactics would not be anticipated and defenses adapted from past encounters by the Borg. The Borg's own manipulation of genetics by introducing cybernetic implants that allow drones to re-energize organic tissue with direct energy, light speed adaption to strategy, and regenerating ships speaks to pretty awesome technology. That and their sheer brutality, would make them too formidable in a real war.
I think the Dominion would have to get real lucky.