Advice making simple solar shed vent from salvaged panel
That was a long question :-)
You are not out of your mind and think in the right direction.

In fact you can get twice much amps you need for your fan.
So yes, get one of these modules, and add some extra capacitors on the output (some 10'000 uF) to help it deal with FAN's pulsed current consumption &amp; startup current.

You can even get way bigger fan, cap will help to start it and it will work even if you have just 30-40% of the power it needs. You just need separate switch for DCDC board and FAN - so that you turn DCDC first, and after 1 second - FAN (so that cap is charged).

And finally, direct sun gives much much more juice than 60 Watt lamp. So you really need more powerful fan &amp; more powerful DCDC.