I'm not sure what your body posture looked like when this was happening. If you were on the nose of your saddle leaning way forward, chest on/near the handlebar, then you did everything you could short of carrying more speed before the hills. That can help a lot, although deep sand tends to kill momentum quickly.

Tire model and inflation pressure are all that you have left if you can't fit a wider tire. What tire is on the back, what do you weigh, and what pressure was it inflated to?