The founders only offered concessions to "the solids" because they were beaten during the Dominion war. It would be believable that they would splice a klingon/Jem hadar hybrid if the Dominion had become allies with the Federation and Klingon empire since that time.First of all, welcome to the board, @Captain Graham!
As for your post: What’s the reason why any of that couldn’t have changed in the 800 odd years since we last saw a Jem’Hadar?
But they were generally not a people that liked to change the status quo at all and would not hesitate to genocide entire worlds if they provoked their wrath for thousands of years even before DS9.