This work is dedicated to a refined error analysis of the high-order transparent boundary conditions introduced in the companion work [8] for the weighted wave equation on a fractal tree. The construction of such boundary conditions relies on truncating a meromorphic series that approximate the symbol of the Dirichlet-to-Neumann operator. The error induced by the truncation depends on the behaviour eigenvalues and the eigenfunctions of the weighted Laplacian on a self-similar metric tree. In this work we quantify this error by computing the asymptotics for eigenvalues and bounds for Neumann traces of the eigenfunctions. We prove sharpness of the obtained bounds for a class of self-similar trees.