Try wrapping the foam in an egg shell mattress top, wrap put layers of quilting on top of the foam, or look for foam that is not as dense. I like the look of one seat cushion and wouldn't go to another configuration.

Just change out the foam. Putting a bunch of soft material over it won't really help and your cushion cover won't fit properly. It's a nice sofa, worth the extra investment. Good for you for building it.

Depending on how much you want to invest in a new cushion I would have an upholster make a seat cushion for your sofa. They will be able to make a cushion with interior springs which will be like a factory sofa cushion. They will even cover it in the fabric of your choice. But it will cost you. But you made it yourself so I think it would be worth the investment.

i think its worth *trying* vicki's suggestion of two layers, you could even try a less dense foam on top of your firm one.