As a little exercise in integration i figured the actual energy
dissipation during the capacitor's charging.

The dissipation indeed starts at around 53W, however after 300uS it down
to 0.5 watts. The total energy dissipated during this time is 0.00355
Joules. Vishay rate their 805 SMT thick film devices for an overload of 1A
for 5 seconds. That is 5 Joules. However: assuming all of the energy is
dissipated in the resitive element which is pretty accurate, and based on
some rather broad assumptions (resitive element is 150um thick, made of
RuO2, based on some guestimates of the specific heat capacity) the
temperature rise of the resitive element is to the order of 200 degrees
centigrade, and this could be easily be twice that around the area where
the resistor was trimmed, so maybee final temperature of around 450
degrees for small areas of the element. The situation is pretty marginal,
however this is the worst case - if the FET absorbed some of this to
spread the dissipation over slightly longer it would be fine - or of the
reistor is increased towards about 15 ohms it would be within its ratings.
The other consideration would be thermal shock - even if it withstood it
once it much fail after a few thousand cycles.

Best of luck

Alex

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