I’ve never seen that type of suspension frame before.
The advice I can offer about this is that all the springs in a system of multiple springs and masses need to be balanced in such a way that a single spring somewhere is not overwhelmed by the dynamic motions of the system.
That is to say, you want them all to be stiff enough that one spring doesn’t tend to bottom out while the other ones are still in the middle of their travel somewhere.
Ideally, under extreme motion, the springs will all bottom out at the same time.
You might find yourself adding additional springs or springs inside of springs, to try & double up the pressure.
But it’s a complex system, and the weight of the rider can be a huge factor in what’s acceptable or not. That’s something you’re gonna have to work out by trial and error. Fortunately, in the environment of a cruiser, all of these things are less critical.
I’m wondering about the details of that suspension device behind the seat post. Have you had that part of the bike apart yet?