My preference is to use VCAs for more VCA stuff, if you are mixing the 2x Stereo outs you'd take up all 4 chanels of Quad VCA and end up with Mono. Happy Nerding make 4xMix which is a compact 4hp mixer - https://modulargrid.net/e/happy-nerding-4x-stereo-mix
If you want something with a bit more scope then the DPW Designs M1 adds stereo panning and a bunch of other things https://modulargrid.net/e/dpw-design-m1-mix but its more HP
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.