Turtle tanks can be completely full, they can handle deep water. I would only keep the water lower if that is the best option for getting them a proper basking area.
Does it have a built-in overflow or just drilled with bulkheads?
I would not run just a sump, it won't give you enough of the right filtration.The best option in my experience would definitely be to run a pump from the sump to a pond filter that has a backwash option. It is not the cheapest option, but when it comes to filtering 250 gallons of turtle tank, it is the only option. It will allow you to clean the filter without touching it. Just turn off the pump, turn the lever, turn the pump back on and let it run until the water comes out clear again, turn the pump off, turn the lever back, and turn the pump back on, that is it.