Rear seems too narrow. Should be closer to 53″ probably.
Could be that you are too narrow right now and the kart is lifting too hard initially and overloading the outside tire, causing it to lose traction and slide mid-corner.
I never keep track of tire temps, but how are you measuring your tire temps to determine that they aren’t hot enough? If the kart is sliding the rear, the rear tires should be overheating, which would be the opposite of what you’re describing. They would show a corn-row type graining pattern with slight cupping/tear on the inner shoulder if they were indeed sliding and overheating.
How big is your driver, what class is this, and what tire are you on? Can the driver feel the inside rear wheel lift on turn-in? Does the kart turn-in appropriately? Any understeer?