The bottom of the chip in QFP package needs to be grounded. Is the solder paste soldered by reflow or wave soldering?

One, if you have enough manpower, you can arrange someone to put solder paste on the front of the mounter, but the thickness of the solder paste should be enough.

Second, open the ladder steel mesh, but this has great limitations.

From your description, I still think it would be better to use some solder paste, because the difference between 0.5 and 0. 15 is too big, and there may be other problems in opening stepped steel plates.