Increasing the number of allowable overruns is a good approach when there is one task that overruns during the model startup. This doesn’t seem like the constraint you’re observing in your model.
If I am interpreting your results correctly, with a 5us step size and 4.2 us max execution time, you are still seeing overruns. This may indicate that the lower priority tasks in your model are the cause of the overrun. The reported cycle time only reflects the load of the highest priority default task with the shortest step size. The base task timing is limiting in most models (and the only timing constraint in single-tasking models) but perhaps not in yours. I would consider the execution time and complexity of your lower priority tasks, especially considering that the low priority tasks can execute when the base task is not executing.
If you post your model I can make more specific recommendations.