Use two powered USB hubs as your Macs seem to have limited connections.
Connect all players to the first hub. USB1 of the mixer as well.
Connect USB2 of the mixer to the second hub. When handover is needed, move the non-playing player(s) over to the second hub as well.
Least amount of wiring is a utopia, because that’s the nature of having separate units.
Dont forget that the x1850 can accept 2 usb connections by itself, and you don’t need a SC6000 connected to have Serato send audio to a soundcard. You could play the first few tracks using keyboard and mouse, and then connect the SC6000s while you are playing?
But no, messing with connections in a booth during a set is never straightforward. Not using Denon, not using Pioneer, not using Traktor. Actually, it wasn’t straightforward in the vinyl era either, because you had to remove a few of your crates to make place for the crates of the DJ after you, but still play a few songs…
Daisy chaining the players to each other is another way.
Meaning only one usb to switch to the next laptop
Connect usb of player 1 to player 2
Connect player usb to laptop 1
When it’s time to switch transfer the usb to laptop 2
Mix in
It’s straight forward
That’s presuming the mixer is not in the chain, otherwise you’ll loose audio.
The serato rule is to always connect mixer/soundcard directly to laptop.