I ended up buying a used Long Haul Trucker frame and moving the good parts from the build above (pedals, wheels, tires, shifters, chain and cassette) over to that. The RockHopper conversion was great as a transitional bike until I stumbled across the LHT on craigslist. Now I'm in the process of turning it back into a mountain bike.

The big advantage of a conversion like this is that it's cheaper to buy a few parts and see how you like this type of bike than it would be to buy a better frame and the parts to go with it, and if you shop wisely you can re-purpose all of the good bits later if you do decide to upgrade. I spent around $600 on the RockHopper, mostly on the wheels and tires. It probably wasn't a $600 bike, but now I've got about $850 in my LHT and it would be hard to get a better bike for the purpose in that price range. Meanwhile, I still had most of the parts to re-build the RockHopper as a $100 MTB (though being a tinkerer that's not quite what I've done).

Or, you might end up loving it and just keeping it the way it ends up. Either way, it's fun to monkey around with things like this.

If you've already got a 520, I'd say trekking bars are a good next step to explore.

Alternatively, you could try bar ends. Like you, I have wrist pain when I use a flat bar for too long. I've been able to get pretty good comfort with Profile Design Boxer bar ends. The main downside is that you have to move your hands out of the comfortable position to get at the brakes and shifters. That's really not as much of a problem as it sounds like it would be.