- Better thermal performance (you can model that in Hyperlynx). Even copper area on inner layers help dissipate heat.
- Filled or blind vias are preferred so that the solder doesn't wick through the vias resulting in a poor solder joint to the QFN thermal pad. A good designer can make thru hole vias work; it is a trade off between via area, surface area, and the solder paste mask. More via area gives a better thermal path but also wicks away more solder.
Solder paste mask will not work in most cases ,this will affect the solder joint on thermal pad. Better to do the filled vias with Epoxy.
Thanks and Regards
Thermal PAD on the Bottom Layer is not necessary.
You can if it is a single sided board.
CON - On double sided board you will be sacrificing lot of space.
I prefer to NOT make it part of the footprint/decal. I like to draw these in as copper pour shapes as necessary.
If you want to "group" the shape with the QFN and circuit, you can define it as a stand-alone Reuse Block. That way components, traces, vias and shapes of a circuit can be moved around together.
Consider making the bottom side shape "void" of solder mask to reduce the thermal resistivity. If you can afford it, go with ENIG to avoid surface oxidation (and increased thermal resistivity).