1. Vorta are immune to poison is not the same thing as been immune to nanoprobes and I can't remember anything been said similar for the Jem'Hadar. They don't eat or drink so the chances to poison them in limited to tainting their supply of white as Damar suggested if they couldn't get the wormhole open and supplies became a significant issue. So that means you can "poison" the Jem'Hadar.
2. Just because you can jam the Federation's sensors means little to the Borg. The Borg are more advanced then the Federation.
4. There is nothing stopping the Borg from just capturing a Jem'Hadar vessel and getting the technology information that way. They have a powerful tractor beam that easily caught the Saratoga and the Defiant, which are bigger then a Jem'Hadar attack fighter. Or capturing the technology from the Federation, Klingons or Romulans. The Borg could learn a lot of the necessary info from external sources.
6. Assimilation is fair faster then growing clones. Picard turned into Locutus in a matter of hours, Hawk went bad in a matter of minutes. All the Borg need to do is stop off at any inhabited world and they can instantly reinforce their number of drones.
7. In the Dominion War for the most part it didn't even seem like anyone was using shields. We saw multiple instances of ships taking one or two hits and blowing up straight away.
One point not mentioned is that the Borg have transwarp, which is a big advantage. If the Borg fought the Dominion in a similar way that they have the Federation, the Dominion will most likely win. But if they threw their weight behind a full scale assault their larger, faster and more powerful ships would win the day. They could outrun Dominion fleets, strike at Dominion industry, weaken it that way. Mess with the supply of white and the Dominion would be in significant trouble. Lack of white = rubbish Jem'Hadar. The Dominion would help destroy itself with Jem'Hadar going nuts and killing each other.