The p-benzo is not going to dissolve all initially, no matter what you do. Just stir, as the reaction progresses it will be gradually consumed and everything will go in solution.
I think the Wacker temperature is not that important. It is usually exothermic, the peak temperature tends to be about 40 C or so. But I and other bees have seen cooler peak temperatures, both with high and low yields. If it is cool, I like to apply heat and to keep it at circa 40 C, but I don't think it is that important. You don't even need a bath for that, just make your BF touch the hot plate, set it to low, watch the temperature.
EDITED POST : P-benzo is essentially insoluble in water. The DMF helps to dissolve it, but it will not dissolve completely. P-benzo is much more soluble in non-polar solvents like DCM and benzene.