That's a big ask, to have a bike that is well-suited for both commuting and mountain-biking. You'll need to compromise on being somewhat inefficient for commuting, and somewhat limited in what kinds of trails you can easily ride.

There is a category of bike called a 'gravel-grinder' (also similar to an 'endurance bike') that would probably land pretty much in the sweet spot, but you'd probably have drop-bars, and I'm guessing if you really want a mountain bike you wouldn't like that.

Personally, I'd recommend you stretch your budget and get a Surly Krampus. As a few up there suggested, you'll have nice wide tires for sand. It won't be the most efficient rider on the street, but it won't care about bad roads at all, and your distance of 7km is not too long.

In terms of simplicity, it is about as simple as it gets. Rigid fork, so no need to maintain a suspension fork. 1x drivetrain, so only 1 shifter and 1 derailleur to worry about, instead of two of each. Brakes are hydraulic, that might take some learning, but shouldn't be a huge problem. If you can find a used original model (green) then the disk brakes are mechanical, which is even easier -- and you might be able to get it under $1000. The MSRP of the 2017 model is $1500.

Here's a (long) article about what got upgraded in the 2017 model.