Great that you're enjoying riding!
As far as a tired back leg, it sounds like you might be sitting in the backseat too much and/or skid turning, where you use your back leg like a rudder. It's not necessarily a bad thing, but it will tire your back leg out quicker. Try balancing out your weight and driving through turns with your front leg. You should also take a lesson to learn proper carving technique. As you gain more confidence, your skid turns should subside, but again - proper lessons will expedite the process.
You should absolutely practice switch riding. Its the one regret I have when I first started - I didn't do enough switch riding. Because you're learning the basic mechanics for your dominant foot-forward, it will be easier to translate to switch. It will make you a better rider all-around. Pick a run or two each day and do the whole run switch. It requires discipline, so stick with it.
Butters are fun! Practice with the board at a standstill, and slowly work up to moving butters. IMO butters should come secondary to working on the above stuff, but hey - have fun and do what you want.