We investigate multiple-$Q$ magnetic states in centrosymmetric itinerant magnets, focusing on the competition between two inequivalent groups of ordering wave vectors on a triangular lattice. Using an effective spin model with bilinear and biquadratic interactions, we systematically study the effects of momentum-space frustration, the biquadratic interaction, and an external magnetic field by simulated annealing and classical Monte Carlo simulations. For weak momentum-space frustration, the system stabilizes conventional triple-$Q$ skyrmion crystals with skyrmion numbers of one and two. In contrast, strong momentum-space frustration destabilizes these topological states and instead stabilizes mixed multiple-$Q$ states composed of inequivalent ordering wave-vector sectors, including noncoplanar but topologically trivial triple-$Q$ and quadruple-$Q$ states. We further show that increasing the biquadratic interaction promotes higher-order multiple-$Q$ superpositions and leads to the emergence of unconventional sextuple-$Q$ states. Most notably, near zero magnetic field, the competition between the two symmetry-inequivalent wave-vector sectors stabilizes an isotropic sextuple-$Q$ state with finite uniform scalar spin chirality, in sharp contrast to the chirality-compensated sextuple-$Q$ state formed from a single symmetry-related wave-vector star. Our results demonstrate that the interplay between momentum-space frustration and biquadratic interactions provides a route toward exotic higher-order multiple-$Q$ states beyond conventional skyrmion crystals in centrosymmetric frustrated magnets.