"should we move things forward to be together sooner, or struggle on through for another couple of years trying to balance it? "
You've already to an extent have experience in balancing the relationship with professional/educational endeavors. This is another stage of your life together. At one point sooner or later, one of you would have to make a compromise on behalf of the other. That is a testing time for any relationship (even marriages) At this time, both of you are pursuing other things and yes, the distance can take a toll on the relationship. But, to what extent and to what outcome would depend on both of you. It is also something that can not be predicted yet until you've tried it.
As far as getting married sooner than later, you'd arrive at that decision based on your faith in each other. If you believe that it is just a formality and no matter what comes your way both will work together to manage it, then you can do as you've planned. Keep in mind that even if you're married unexpected things will still happen in your life (the same way as when you're not married) The two of you will have to discuss what this would mean for the preparation and other individuals who may have been involved in helping with the wedding plans. If you trust each other and waiting is something you're both willing to do, then you can post pone it. It largely depends on what being married means to both of you (an official commitment, proof of your love, trust, security, etc) It is this that you've got to figure out and act accordingly. Asking yourself what would marriage actually mean in the larger scheme of things.