1-getting crushed on climbs of less than 15 minutes has more to do with your legs and gearing than your bike.
2-21 lbs. is not a bad weight at all.
3-the top two places to lose weight on the bike are the crank set/bottom bracket and the wheel set.
4-Your wheel set is a freewheel type, so upgrading it won't save much weight.
4a-however, a bearing repack and a true/tension of each wheel will bring them alive and make them seem new.
5-You could get a newer seat post, but it won't save a lot of weight.
If you addressed each issue and spent money to lighten each issue, you'd be better off buying a good used 5-10 year old bike. It won't be as cool as your bike. However, there are ways to climb better and be faster on your bike, without dipping into the wallet as deep.
1-Fit. Make sure you have it correctly fitted. Why waste power due to improper fit?
2-Fitness. Can you lose the 3-4 lbs. that would come off your bike? Improve the engine. Try running a bit.
3-Gearing. Your climbing may benefit from a wider range in the back, or a smaller front chain ring.
4-Clipless pedals. The jury is out on these, as far as a huge advantage, but they help develop a smoother stroke.
5-True/Tension the wheels, get the hubs repacked and adjusted. Like buying new ones.
All the above make your bike a smoother, more enjoyable ride, whether climbing or not.
When it reaches it's full potential, you'll still be looking for yours. If it ever becomes the obstacle to speed, then change bikes.
I state these things as opinions, though they may sound as if they're facts. I believe them to be true.