New Couch Cushions Too Soft
If everything else about the sofa is just right and you made sliipcovers that i assume you like too perhaps you could get new cushions to fit your slipcovers that would be better.
New couch cushions too soft. Repeat the process until the cushions reach the desired feel. Insert more foam spreading it around to make it smooth and even. I believe that the actual problem is in the frame allowing the springs to sag. Put the cushions back on the couch and test their softness.
The company rep did come out and authorized new cushions for the sofa. The cushions and pillows of a new couch will inevitably be a bit stiffer than the ones you are replacing. You can put it around the current couch cushion but inside the pillowcase. Take all the cushions off the couch.
Of course you are not going to toss out your perfectly good couch merely because it is too soft. It did not fix the problem and they refused to do anything else. If your cushions are filled with loose stuffing stuff them with polyester fiberfill. I have had the couch too long to return but assure you that t will never buy anything from slickley again.
Instead give a quick fix to the sag with a piece of plywood. The stiff cushions can make the simple act of relaxing on the couch a bit of a pain literally. New sofa cushions too firm. If it is badly worn you may need to replace the foam.
I think it would be worth consulting with an upholstery business to ask about this.