So here is what I've mananged to research so far:
1. Here is post from a russian forum where a mechanic has managed to install air struts on a i3. I wrote him and he confirmed that the ride became smoother and that he could send me the corresponding parts so that I install them in my city:
https://www.drive2.com/c/584246395213461185/
2. This company seems to produce Air Struts / Suspension for i3:
https://www.f2suspension.com/product...01-air-struts/
I am going to write them, asking more details about what else's included in the kit (for example does it come with a compressor as well, some sensors that detects the speed, car rolling position, etc in order to adapt the suspension to the ride).
As far as I'm informed many aftermarket air suspensions are made for pimping the ride, so that the user can lift and lower the car, and not orietend towards making the ride smother or sportier depeding on the needs.
3. In the meanwhile have found out that wheels can also be retrofited with R18 with wider tires, which will require wheel spacers and the arcs from i3s.
Would be great if you guys can dig on the internet more info and to share here the solutions to i3's super-stiff ride.
Cheers!
Julian