My first piece of advice would be that if you're enjoying the bike, and it's meeting your needs, there is no need for an upgrade.

Wheels do make a significant improvement, but not in ways you'd expect. Sure, lighter wheels accelerate faster, but that rotational inertia only applies to changes in momentum, so you don't get the compounding improvement to help you get up hills. Where they'll help the most is comfort. Wider rims and wider tires will let you run lower pressures, which greatly improves comfort and makes a marginal improvement in rolling resistance.

However, sometimes a new bike gives you a big boost in motivation, which is the best improvement of all. If your state of mind tells you to ride more and work harder, you'll make the most improvements to what matters most, the human-engine. You don't need a new bike to motivate you, though.