Something just struck me that probably be worth considering- the weight off the bike. And another reason to opt for a backpack over panniers for the commute, anyway. And why is that you ask? Take a look at the pics in the OP. He's planning on cutting across pasture land. Which means unless he obtains a key to gates from the property owner(s), he's gonna have to lift that bike up over a barbed wire fence or the metal gate. And the backpack lends itself to fence jumping than panniers do.

Concentrate on what works for the commute. The commuter can be pressed into a weekend tourer- and rereading the OP, sounds like he'd more likely ride through a National Park than go cruise Rt. 66...

At least the OP does have a budget where he can obtain something a bit lighter than entry level as far as the mtb's go, though if taking weight into consideration, he may want to reconsider a dual sport hybrid.