Composite solid electrolytes are gaining interest regarding their use in Li-metal solid-state batteries. Although high ceramic content improves the electrochemical stability of ceramic-rich composite separators (C-SCE), the polymeric matrix also plays a vital role. In the first generation of C-SCE separators with a PEO-based matrix, the addition of 90–95 wt% of Li6.45Al0.05La3Zr1.6Ta0.4O12 (LLZO) does not make C-SCE stable for cell cycling with high-voltage (HV) cathodes. For the next iteration, the objective was to find an HV-stable polymeric matrix for C-SCEs. Herein, we report results on optimizing C-SCE separators with different ceramics and polymers which can craft the system towards better stability with NMC622-based composite cathodes. Both LLZO and Li1.3Al0.3Ti1.7(PO4)3 (LATP) were utilized as ceramic components in C-SCE separators. Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDDA-TFSI) and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) were used as polymers in the “polymer/LiTFSI/plasticizer”-based matrix. The initial phase of the selection criteria for the separator matrix involved assessing mechanical stability and ionic conductivity. Two optimized separator formulations were then tested for their electrochemical stability with both Li metal and HV composite cathodes. The results showed that Li/NMC622 cells with LP70_PVDF_HFP and LZ70_PDDA-TFSI separators exhibited more stable cycling performance compared to those with LZ90_PEO300k-based separators.