Nice looking welds.
...try doing a butt joint and try the break the weld...
I agree. This will show you alot. Try playing with the machine, Increase/Decrease amperage, speed, wire feed. This will help you learn the machine. Anything over 1/4", or if you can get to it, weld both sides.
No professional here, everytime I have a nice lookiing weld, there is little penetration. So I always just 'burn it in' and let the grinder smooth it out.(I need more practice) Only have had a few welds break(less than 5 in 20+ years), most of the time(99.9%) it breaks at a factory weld, so I don't feel bad about my welds. Good luck and keep practicing.