Four 3mm washers for the tops and bottoms of the axle holders can reduce the friction between the upper and lower suspension and the axle holder, to make the steering a little smoother.
If I had nylon washers in that size I’d use them instead.
You could print trimmer line. Someone in the neightborhood has always a couple of meters lying around.
In reality it only makes sense to print the larger plastic parts 
If you want durability for wings etc. the current filaments can’t match it yet. I myself is going to try to modify the build to use a Tamiya F104 carbon drive shaft instead of the plastic one. I have bend one in alloy so i do not believe 3d printed parts are good for this task.
Did you try trimmer line / nylon instead? its horizontal strength is awesome
Well the drive shafts in solid ABS have been remarkably durable (same goes for the spur gear). Since switching to 100% infill (which is still lighter than metal) they have withstood a surprising amount of abuse with no failures so far. the only one I broke was dropping the car off a bench. A CF F!04 driveshaft should arrive here Friday for testing. Then we shall test race original stock “Plastic Fantastic:” (which is going pretty fast currently, faster than my reflexes) vs. metal, CF, and plastic “Hopup Opushonzu” (which with a metal pinion gear will do some more strenuous testing of the thick, but bendy/resilient and durable ABS driveshaft/spur-gear imminently).