Me, I would have just bought some add-a-leafs or some real 3/4 ton truck springs... that's like half the total springs in my Dodge's pack....
On the subject of the shims, rot will never be an issue because when the pack compresses and flexes hard that bottom spring a couple times, those shims will start to crack in two and finally fall out.
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Glad to see you are looking at options and discussing your ideas here in this forum. Lots of good ideas and critiques.
One thing to remember, screws (unless you purchase higher grades) are used for bonding, not support. They are great for providing clamping/pressure when bonding/gluing surfaces together. They have no shear force capabilities.
When attaching items together, use steel nails, joist nails, bolts or gang nailing plates (like joist hangers and hurricane clips).
The best way to demonstrate shear force bonding, take a piece of plywood, screw a deck screw (or drywall screw from what I can see you are using) through this board and take a nail, same length of screw or longer and drive it through the same board. Take a hammer and strike the screw and then the nail on the side of them and watch what happens. Yep, the screw will fracture/break before it bends like the nail bends. This is what is happening between two boards that are attached with screws when the board flexes. I have seen too many DIY'ers build decks with screws where they shouldn't. Be careful and things will last a long time for you and not end up in the middle of the road, behind you.