The Founders wouldn't be able to make the Jem'Hadaar impervious to assimilation.
Temporarily it might do the trick to alter their DNA, however, the Borg would find a way around it on their own eventually because the nanpoprobes adapt and rewrite the DNA.
Which is exactly why they steamrolled Species 8472... oh wait, they couldn't be assimilated which meant the Borg couldn't adapt and were on the verge of being exterminated if Voyager hadn't given them a hand.
Species 8472 was from a different dimension which was also highly evolved/resistant in not just biological, but technological aspects.
None of the Milky Way species would be able to compare to 8472.
Maybe the Voth from 'Distant Origin' episode would be able to come a bit closer in technological areas ... but biological ... possible but unlikely.
The Founders technology is practically the same as the Federations.
Their genetic manipulation tech is probably also on the level of the Feds, but the main difference is, the Founders USE it to their advantage for creation of super-soldiers while the Feds do not.
The Founders would probably rank as 'amateurs' when compared to 8472 in genetic manipulation technologies.
One other thing ... yes the Borg were in a losing battle, BUT we don't know if they would have lost for certain.
There is a possibility they would be able to adapt on their own (at least to the weapons partially) given enough time or just as the end grew near.
8472 is essentially what the Federation was to the Borg when they first encountered them ... only much more technologically/biologically developed with a higher resistance quotient when it comes to assimilation.
Give it time and the Borg will find a way around that.