With self drilling screw should I use a drill or a driver

That is how it should work. The drill is designed so that less pressure on the trigger means less power to the drill. It could damage the driver if you were to keep it in that state of trying to turn and failing for an extremely extended period, but not if its momentary like you describe.

In short - good question but you're worrying too much. :)

Oh - yes - by the way - actually this is what it was designed for - exactly so. It's designed to apply torque to bolts and screws and drill bits.

One option especially suitable for this scenario (outdoors, often drilling above your head, with the tube bending and vibrating, using handheld drill) is using special drill bits tailored for fast drilling into sheet metal. The market has numerous offers, including Irwin Turbo Max, DeWalt Extreme 2 and Ruko Fast Cut - the idea behind all those is that the drill bit looks similar to a drill bit for wood and the small tip of the drill bit helps the drill bit dive into thin metal.

One important thing is that you should not proceed to drilling the second wall until you retracted the bit from the hole and checked that there's no metal on the tip. Otherwise the following can happen: the drill bit will burst through the first wall and the metal which the tip was facing will remain on the tip and prevent the tip from contacting the second wall.

Beware of the following downsides:

• these are next to impossible to sharpen in your workshop - prepare to either throw them out or grind them into "regular" shape bits once they get dull
• the price is notably higher compared to "regular" bits
• the "metal on the tip" problem described above
• not that easy to find in an average store, so you should shop in advance