I wonder what is actually inside the driver module?
I have seen some very nasty, cheap, 240VAC LED driver modules before that looked about the same size, and all that was inside them was simply a 0.47uF 250V polymer capacitor (China-X2 rated) a resistor and an antiparallel diode across the LED. Absolutely no galvanic isolation or proper current regulation at all.
Also, with regards to thermal interface grease you do not need just a big blob of thermal grease in the middle of the heatsink, especially if that blob only ends up covering about 1/4 of the contact area between the device and the heatsink. You don't need much grease, just a very thin smear. But it should be a thin smear spread out across the entire contact area between the device and the heatsink. Ideally, you want a device-to-heatsink contact as much as possible, with direct thermal conduction across the metal-to-metal contact (or epoxy, ceramic or whatever the device package is) in contact with the heatsink across as much mating flat area as possible.
But in practice the two surfaces aren't perfectly flat, although they should ideally be machined as flat as practical. Therefore there will always be minute surface imperfections that are filled with air, where the two surfaces don't quite touch. The purpose of thermal grease is to fill in those minute surface imperfections to create a perfectly flat surface on each of the two mating surfaces. This material has a higher thermal conductivity than air, so it's superior to an air gap, but its thermal conductivity is worse than metal, so it's better to have exposed metal as much as you can and only allow the grease to fill in the flat surface with a really thin amount, just where it's needed, and that's how thermal grease should be applied.