Well F1 has volume faders plus filter knobs for four “stacks” of remix slots, so maybe you don’t need master volume and EQ controls for remix deck. I’d suggest that you get the F1 and try if you really want master volume and EQ controls. And if you want those (like I do ) you can probably midimap Z1 so that you can control decks C and D with it, but if you don’t need Z1’s internal soundcard, I’d suggest to at least check out Behringer CMD MM-1, as it offers mixer controls for four decks, has USB hub and is cheaper than Z1. But yes, it has no built-in soundcard and is not iOs compatible…
Ideally the remix decks need a dedicated channel, as you have an S2 it is still possible to use the remix decks in internal mode, however they would share an audio channel with your track decks. You can still control the Volume & EQ on them regardless.