Hello Zoran
My 50 cents
Front wheels:
The trike front wheels I'm recently using on my Velocar have a 12mm Axles of high grade steel. (12.9).
Never had problems with sideward stability, even with 24" wheels.
The strength of the wheel/kingpin connection depends also on the outer diameter of the washers, which are stacked between the backing plate of the drum and the kingpin mount.
If the axle bolt is properly tightened, the bolt itself is only stressed with tension, no bending forces at all.
The backing plate, the washer and the kingpin mount are forming a virtual tube around the bolt and taking up the pressure.
Rear wheels:
I would use BMX wheels, but mostly no disc mount on these wheels, so I have a brake problem.
I'm toying with the idea of using Tuff Wheels, maybe a version with disc mount is now available.
A follower of Paul Elkins used such wheels.
Klick for a YT-video of his car
Cassette wheels are a bad choice, because of the asymmetrical spokes and the spoke flanges very close together
Although I used such wheels on my Velocar for two years without problems (always cornered very carefully) I won't recommend such wheels.
On the other hand, look at all these old Mochets, with seemingly normal wheels, sometimes 26".
Seems to be rigid enough.
Another solution would be self constructed hubs.
With 3D-CAD you can design your own hubs.
A lot of businesses around here with CNC milling machines, who are able to produce such things from a data file.
You even don't have to leave the house for this.
Send the file via E-mail and wait until the mail man delivers the parcel is all you have to do.
OK, I don't talk about the money issue
Kai
