Stupid mechanic here who works on automotive body-in-white, not a metallurgist. That looks like a wood screw or sheet-metal screw. There is no way you are going to self-tap 6mm of steel plate, even mild steel, with a screw like that.
Option 1 if you can get to the backside, a proper screw and hex nut with normal everyday M6x1.0 threads. No self-tapping no nothing. Drill a clearance hole, position the nut and screw, tighten, done.
Option 2 if you can't get to the backside, drill and tap a normal M6x1.0 hole and use a normal off-the-shelf M6 screw. Done.
Option 3 if you don't want to drill and tap, weld a weld-nut to the backside and use a normal off-the-shelf M6 screw. Done. We projection-weld weld nuts to the backside of sheet metal parts all the time. In my home shop, I've MIG-welded normal nuts to the backside of steel brackets many times.
Now, over to you. Wny won't these "standard off-the-shelf" solutions work for you?
Self-tapping screws are for sheet metal ductwork in a house. And I've never seen one that long.
I've seen self-tapping screws for ISO metric threads.
Those list use in softer materials ... but I've run across larger sizes of them used in automotive applications. I seem to recall one car that I owned, using M8-ish self-tappers holding the driver and passenger seats down to the floorpan.
