I find clean up only takes a couple of minutes with my Omni 4000, just a flush out with water, followed by thinner and a stiff brush to remove any paint from the paint cup, and the nooks and crannies at the botton of the paint well. A bit of back flushing, and a final flush with warm water, et voila a clean Omni!
Needle lube helps a lot, and I find improves control as well as clean up.
As for paint mixing, I usually A/B at weekends, so paints are mised in 1/4oz jars ready for the next session during the week. I find this ensures consistent results, and no faffing around either, mixing during valuable A/B time.
One final thing, I've found it pays to plan your spraying sessions, ie all items of a certain colour, maybe from several kits, all prepped and ready to spray in one session. This also reduces colour changes, and thus clean up.
Karl