I will describe a mixed-action lattice QCD approach using CLS ensembles with $N_f=2+1$ flavors of $O(a)$-improved sea quarks together with Wilson twisted-mass valence quarks. Once the valence and sea quark masses have been matched, a comparison can be made for results of the mixed action setup with respect to those of the regularization based only on $O(a)$-improved Wilson fermions. Preliminary results for the gradient flow scale $t_0$ from a scale setting procedure will be presented. The extension of this mixed action approach to the charm quark sector will also be described.