[Puke]

some ideas:

build three times as many unarmed escorts and trade them for their DN fleet.

wait for them to either scrap their fleet, or fly it through a few damaging wormholes.

send a ship into the system their fleet is in, and blow up the sun.

yes, the first two suggestions were jokes. if you want a standup fight, build some construction bases at each planet (should take 2 turns if you go cheap on them) and start cranking out a fleet. since you get to build your fleet after they have theirs, you get to tailor yours to be the perfect counter to the technology they are using.

it might help to disrupt their production ability while you do this. in 1.19 the AI is perfectly capable of blowing up stars, so its fair game to do it to them. saves conquering planets, but dont expect them to like you much afterwards.