Lots of black oil/crud at first is a good sign. It's telling you the oil is rinsing the breakin crud from the engine. That'll get lighter in color as the engine breaks in, and there may be less of it as you get your carb adjustments closer as you use it a little more.

Your borrowed fuel. Was it mixed 32:1 or something very close to that? Many are running 40 and 50 to 1. You need to pretty careful of that, especially this early in the break in process. The manf. suggests 32:1 all the time, so it's really important early on....

Gassers are pretty much wide open at 1/3 throttle opening. They're no where near close to being linear.... and as you've now seen/realized that fact is very apparent from the time you land one the first time.

Having a second set of eyes, especially more experienced ones, go over your new plane prior to flying is a good plan. Depending on others to spot your setup issues isn't a very good plan. If you know of something better at least try it. If it works out well for you, then you'll have something to share with others at the field.