We present preliminary results for a scale setting procedure based on a mixed action strategy consisting of Wilson twisted mass valence fermions at maximal twist on CLS ensembles with $N_f=2+1$ flavours of O($a$)-improved Wilson sea quarks. Once the matching of valence and sea quark masses is performed, universality tests are carried out by comparing the continuum-limit results of the mixed action setup to those of the regularisation based solely on O($a$)-improved Wilson fermions. The scale setting uses the pion and kaon decay constants, in units of the gradient flow scale $t_0$, obtained from combining computations with the unitary Wilson action and the mixed action. The proper isolation of ground states as well as the continuum-chiral extrapolations are evaluated through model variation techniques. An update on the determination of $t_0$ will be presented.