I did similar for trailer builds. Keep me posted on the ninjaflex material. I used a soft filiment and tried to print tires. It didn’t turn out well. I forget what material I was using something with the name sun in it. It was flexible but too stiff yet, I didn’t try another print with thinner fill. The outside was fuzzy also. I’m not intending to discourage you but would like to know how it works for you so I can get the right material
. I did smooth the outside with a vapor bath and it did smooth out fairly well. But with my printer and material I was using I could not get small detail like treads to come out. Keep up the great work and lots of pics!!