AussieHard
AussieHardTail
it's gonna be different for different people. I know this first hand now, because I just did my first ride on my sprung solo seat, hard-tailed XS. If anything, the back felt "squishy" due to the 5 inch springs - not so much up and down squishy, as the springs are quite stiff, but rather a sort of side to side squishy wallowing. Gonna try 3 inch springs first, before I sh_t-can it for a solid seat like punkskalar has on that sweet bike in his pic.
The other thing is there are several types of springs and set-ups for sprung seats...so if someone had the energy and the funds, you could knock yourself out figuring out/engineering alternatives. I'm intrigued by a couple of bikes I've seen that use a flat bendy piece of steel kind of like a "leaf spring" suspension for the seat.
That is exactly what they are leaf springs, Some old tractors came out with that set up and some people make their own.
I wonder if the seat mount/hinge point has anything to do with your "Squishy" feel. Some hinges are broader therefore allow less play.
I do remember one build thread saying he had that issue and modded or bushed his.
Others may have suggestions as to what brand or design are butter made than others.