If the o.p. decides to keep their bike (I would not) and simply replace the rear tire (or both!) chain, etc. then taking the bike to a bike co-op or bike kitchen can save them as much as 100% on the labor costs. And the turnaround time for the work may well be 'while you wait'. LBS's have gotten better on their turnaround times, but many still prioritize regular customers and push new work out days, if not weeks. 6 miles one way is a worthy commute. It is (IMO) worthy of a decent commute bike. That says hybrid to me.
You get your more upright seating position and gearing more suited to the purpose. A bike co-op can set you up with a used hybrid for not a lot more than the estimated cost of repairing the HardRock. Double that can put something really nice on the road. I normally ride late model hybrids or race bikes but liked the look of the early mountain bikes. Not for long. I was so slow on the thing I simply couldn't stand it. The suspension, tires and sheer weight just suck the speed out of your legs. There are so many commuters on MTB's these days and I simply do not understand why. At least get some Schwalbe Marathon tires on there. You will take 10 minutes off of a 6 mile ride! Late model or vintage hybrid for the win in this use case. IMO.