A couple of thoughts here for you.
Softer springs will lower ground clearance. Especially with SWMBO in the back.
Have you discovered the spring rating of the original shocks? If not, how will you obtain this? Thereafter do you know which marque and model will fit the Twiz and whether a softer spring is available.
Fitting shock springs can be a challenge.
What will be the effect on the existing damping if you soften the springs? You may cause a wallowing issue which may be worse. Plus weight transfer (nose dive) may create uneven braking.
A small, light car can be very sensitive to alteration.
In my previous comment I said I have replaced the anti roll bar bushes with a softer bush. These have certainly reduced the rigidity of the suspension. And I do recommend these as a cheap(ish) way of improving the stiffness.
The ones from Ministromer are made on a 3D printer. Shopping around garage supplies with the dimensions of the original may work out cheaper and a chance to experiment without what could be experimenting with spring ratings.
I hope these thoughts help. That said I have much respect for @osbrook and his contributions on this and other sites. I would welcome any further input on this matter he may have.