Yes that spring thing will add about 25 mm of extra height once you're seated on it.
On the downside it will make your seat wobblier in all directions. They make the saddle feel like its coming loose and about to fall off. Its an unpleasant feeling, though you should get used to it given enough time.
You can get 600mm seat posts, but the difference is probably not worth the cost.
I too have a folding bike that is functionally too small for me. My plan is to get an engineering firm to fab up a new seatpost from tube, and to use an old-school saddle clamp on the end rather than an integrated saddle clamp. This will require a minimum of operations, being necking down the top, and applying knurling to the lowest part.
Second option is to find a seat post in two parts. You might see these on Unicycles where the rider leg length could be a large range. These tend to have two QR clamps and a substantial length of overlap. I've not found a useful sized one yet. These are also built for bikes with little to no sideways pressure, so the bending forces on the post will be increased.
Third option is a bit reversed - if you fit longer cranks to the BB, then your lower foot will be lower again, effectively increasing your seat post length. However this could get expensive and will only gain you 10mm going from 165 to 175mm cranks. Then there is the increased risk of pedal strike on turns.
A cheap option is to find the shoes you own with the thinnest sole, and make them your riding shoes. This will only give a few millimetres thougj