When does root bending moment start to become important?
Root bending isn't an issue for braced structures using wires or struts.  The root fitting is just a pin joint on most wire and strut braced aircraft, and even if there is a one piece beam going across, like some hang gliders, it's not under significant bending, except at the point where the wires/struts attach outboard.
Root bending only matters on cantilever wings, and the structural considerations and calculations will be the same for any cantilever wing aircraft, depending on the design G limits, and the dimensions and loads involved.  It's a beam with a tension member and a compression member and you work out how strong the top and bottom caps need to be as with any cantilever beam whether it's an RV-6 or a 747.