Can’t be completely sure without trying your method but maybe
If you are going for the spline width method i would find the point to go then create an array that stores the splines and points that i would have to go by to reach fastest but it would go way too complex if branching gets involved.
Can’t you Create a blueprint/C++ with spline and at runtime let the blueprint spawn a NavMesh based on the spline points. I guess that would also solve your problem then for Branching.