You actually can inset faces with a set dimension. I see you haven't applied scale yet, inset will use the scale. In Object Mode, press Ctrl + A and then click Scale. Then in Edit Mode, select the top face, Inset, and enter in the dimension you want to inset from the edge by. In this case, it would be 2.5mm, so type 2.5. Then select center face, and extrude down by -5.
The circle extrusion would be easier to do with Boolean Modifiers. Create a cylinder at the center of the cuboid with radius 15mm, and any length longer than the extrusion depth. Enable snapping with Snap To Vertex on, then move the cylinder and snap the bottom face with the top of the cuboid by hovering cursor over any of the top vertices. Press G then Z to move on the Z axis. Enter -5 to move it down by 5mm.
Now add a Boolean modifier to the cuboid, set to Difference, and select the cylinder.
You can either hide the cylinder and keep to modifier, or press Ctrl+A on the modifier to apply. Keeping the modifier is usually the way to go, because you can stack as many modifiers as you want, then turn them off or on if needed (This is also known as non-destructive modelling). If you export an object, all the modifiers are defaulted to automatically apply.