Magic Mirror fire hazard.
I have a few modules installed aside from the default ones.
Well, it really depends on the modules themselves. You could have 10 that have little affect on the Pi. However, you could have 1 or two that tax the Pi to it’s limits.
I do have a heatsink but no fan. So how does the fan work? Is it constantly on or do you have it programmed to turn on after a certain temp.
You can do it either way. Personally, I have mine on all the time. It keeps my Pi at a nearly constant 42°C. I prefer that to the temperature swings between it being on and off.