The cook time and temperature I need is not an option on the crockpot

I, like you, assumed that a slow cooker would have a temperature regulator. I took mine apart, in a bid to find the thermostat and adjust it -- and discovered that it does not have one.

Cheap slow cookers contain an heating element which delivers a constant low heat, and all you can do is choose between two or three levels of power. It does not stop supplying that heat when the contents reach a particular temperature. It just keeps pumping that heat into your food.

This means that -- as long as less heat is lost out of the cooker walls/lid than the element is putting in, the contents will keep getting hotter until they reach boiling point, whereupon the energy will instead go into turning liquid into steam.

I don't know whether more expensive slow cookers have a proper thermostat. It would be good to know.

The answer depends on the type of cut. If you have a tender cut of meat then there's no reason to cook it any longer than then desired doneness. If you are using a tough cut then there's lots of collagen that needs to be broken down, and that requires moisture and time. You want to cook it until all the collagen is broken down as that will make the meat tender. That may take 2 hours, it may take 6, it depends on the cut and the thickness.

I have a similar experience with my slow cooker, it goes way too fast. To get around this I've ended up putting in big chunks of meat and veggies because anything else is completely nuked after 8 hours! I've also thought about using a light timer to have it start 3 hours after I leave home.