Check the vent thru-hull and tank fitting
I agree with your diagnosis--your tank vent is blocked. There's an old saying, "when you find yourself in a hole, stop digging." The same logic should be applied to the toilet: when it becomes hard to flush, stop flushing...'cuz you're just lucky that continuing to pump the toilet against increasing backpressure has only resulted in some squirts out the system and not a ruptured tank. Do not use the toilet again or attempt to pump out the tank until the vent has been cleared. The two most likely places for a blockage in the holding tank vent are the thru-hull and the vent line connection at the tank. That's especially true if waste has been allowed to overflow out the vent, either due to heeling or overfilling the tank. Both should be checked and manually cleaned out. To prevent it from happening in the future, the thru-hull should be checked regularly and blasted out with water every time you wash the boat...backflush the whole vent line after each pumpout.
