The backside is the last pic I have in my initial post. Too much metal or look about right?
For a beginner welder, great job!
But with that said, if you were to hand that example into say an Icar certification test, it would fail.
A properly welded joint seem is stronger then the surrounding parent metal and when destructive tested the weld will remain intact while the parent metal will tear.
To me, it appears you have excess penetration on the backside, but the killer is that you still have areas that are not fully welded on the back. You can clearly see the root gap.
I still say turn the wire speed down a touch but turn the heat up. Try when you make your welds, stand your gun perpendicular to the metal, then angle the gun back at the weld at about 60 degrees so the nozzle is pointing at the old weld. This will tend to push the heat and weld back into the previous weld
When welding thin gauge metal, you should strive for NO too as little as small gap as possible. A gap is harder to weld, shrinks the panel more, if one can't get 100% penetration on 18-20 gauge metal with no gap then you need more practice.
The curve you see is from weld distortion. Just like the shrinking disc on how it shrinks metal via heat, welding does the same thing. The localized heated metal is trapped by cooler much stronger surrounding metal and as it cools it has no where to go but collapse into itself making it thicker.
The ONLY way to correct that weld distortion is to reverse it. Shrinking caused the distortion so stretching is what will correct it.