You're relying too heavily on the elite ships. You're doing so much better with the Federation because they're primarily self contained forces with little support value. The Dominion are support ships.
Jem'Hadar frigates are weak, ineffective ships. They underperform everyone else, even when upgraded with Breen weapons, they're still not worth the cost on their own, but they garner a massive level of stat boosting from the elites.
A high level Leviathan will get you 12% weapon cooldown, 10% chance to hit, 20% angular thrust, and 20% weapon range. Non Jem'Hadar ships only gain 7% weapon cooldown and 10% weapon range. Most of this is from the level 6 ability, Enforced Loyalty is more for the secondary effect of preventing mutiny from Cardassian ships. These are both passives.
A Devastator with Glory To The Founders, also a passive, maxed out will get you 5% weapon cooldown and damage, per nearby Jem'Hadar, to a maximum of 10. Effectively a 50% increase to both weapon cooldown and damage in any decent sized fleet. This is only for Jem'Hadar frigates.
An Enslaver with Victory Is Life does even more, but this one is a a 30 second active. 100% weapon cooldown and damage to Jem'Hadar frigates.
For thirty seconds, a swarm of Jem'Hadar will outperform the Borg. The risk in this is that the Leviathan and Enslaver ultimates both come with a caveat, lose the ship and your Jem'Hadar will suicide.
The Keldon, likewise, is important for overall fleet power, with a 10% weapon cooldown and shield mitigation boost for the fleet, and further Cardassian only improvements.
The trick to becoming a nightmare with the Dominion is getting those high level ships to support the swarm of frigates. To this end, you have a very strong defense.
Your trade ports take 8 logistics, restore shields and antimatter on nearby ships, and are otherwise a major defensive installation with durability and damage on par with a level 1 starbase.
Your weapon platforms, while initially terrible, are easily upgraded by utilizing central powering. Two Will of the Founders, with Power Distribution Networking researched, will give your platforms an extra 2500 shield points, a 40 point restore rate, 1000% plasma torpedo cooldown, 200% energy weapon cooldown, and 40% weapon range. They require large fleets to drop, and are severely damaging.
The Dominion are designed for a slow, exceptionally well defended expansion, building up fortifications and an economy to protect and restore your fleet as you go. You can build a powerful defensive position at the same time you're creating your economic infrastructure.
