The calculator I'm using is
Plane Geometry; it's not free but cost only a nominal fee. It has some limitations since it's actually made for gliders. It doesn't take into account any propeller effects, you would have to estimate them yourself. Otherwise, it calculates stability coefficients which can be compared to those of similar models. You might try dihedral at certain airspeeds and would have a pretty good C/G estimate for a desired airspeed. It's not trivial, though.
Washout has no effect on stability but on stall, and of course it might help IF random flow separations are the problem. As a suitable airfoil the true Clark Y comes to mind. (It has still mostly flat bottom so the wing build is easy.) But I wouldn't build a new wing. If and only if the airfoil makes for problems (still to be found out) I would try turbulators (trip strips) first.