> It is very likely that the reason the transistor failed was because of poor cooling, as it is a low cost, high-resistance transistor that is running near its design limits.

This wouldn't be my first guess based on that hole, which was formed more abruptly than a little overheating. Cooling the outside of the package is only going to buy a little more margin, not fix the root problem.

Looking at that schematic snippet, what is up with the capacitance on that gate drive circuit? They want a slower turn on to avoid a popping sound? Maybe it's a reference circuit that worked fine with a larger package or something, and a UF6 doesn't have enough thermal mass? If the turn on/off time is the problem, removing one of C489 or C530 could stop it from happening again.

Or it could be something downstream drawing too much power on that rail or shorting it out. Since you're replacing the part, you might as well just use a FET with a lower resistance and some hope to address this possibility.

FWIW some thinner flux (like a Kester 951) will generally let you fix solder bridges without starting all over.