At 5' below grade I don't think your pump will freeze, assuming you cover the pit with plenty of rigid insulation. Don't build the cover at the top of the pit though. Build it just above the high water line in the pit so that hopefully the cover will be below the frost line or at least close to it. Heat tape can't be submerged so wrapping the pump won't work. Deicing cable might work, but I'd be hesitant to use it in a situation where it was constantly submerged as it is not rated for that. They also make small deicing heaters for bird baths and fish ponds that you could float in the water since keeping that water from freezing will keep the pump from freezing. Deicing cable may work ok in the discharge pipe. Though I'd first try and make the discharge pipe as large as possible - ideally 4 inch - with enough slope so that the water completely drains out by gravity. Also bury the pipe as deep as you can.
My experience is that a 2" pipe will eventually freeze up at around 0 degrees F no matter what the slope is. My 4" pipe has never frozen and is 160 feet long buried 2 to 3 feet underground. It has enough slope to allow the water to run out, but is not overly steep. My 2" pipe is 2 feet long and is a straight vertical drop into the 4" pipe. It is exposed to the air and has frozen after an extended period of 0 degree whether.
I would try and design it so you don't have to use the deicing cable or only use it in extreme whether. It is expensive to operate. Also deicing cable is decent at keeping ice from forming, but not so good at melting ice as it really doesn't generate much heat. So if you go that way remember to plan ahead and plug it in before you have a problem.