Placement of Vias under thermal pads
Thermal Design time.
Since we don't have any idea about the OTHER PCB LAYERS, this cannot be a very exact answer.
However, we can state the thermal resistance of standard thickness copper foil (35 micron, 1.4 mils, 1 ounce per square foot of copper) is 70 degree Centigrade per watt of heat flow PER SQUARE OF FOIL.
For any size square. For any size square.
And given the PCB houses try to electro_plate the Vias to have about the same thickness of copper, we know a via with 1:1 ratio of periphery/depth will also  the same thermal resistance: 70 degree Centigrade per watt.
A via of small diameter, such that periphery/depth of 1:2, will be 140 degree C per watt.
A wide via, maybe 1/16&quot; across thus 3/16 periphery on a 1/16 thick PCB, would have periphery/depth of 3:1, and a thermal resistance of 70/3 or 23 ° C per watt.
WARNING: If you badly chop up the GROUND PLANE into which you are planning to dump this heat, you may create a Product Warantee problem, because hot spots become Warantee problems.