Yeah, Chengou is the most anoying one. It took me more than 20 tries to win that race. Than i switched to Feisar and got it on the 3rd try. I have the impression they are slower when I use Feisar and it's great ship for the Meltdown races.
Keep trying and stay focused. For hard places: use sideshift on the first 90 turn, barrel roll on both jumps. Than on that wide bend pitch nose of the ship down so you have more control. Else should not be so hard. Absorb everything except when you are 2nd and you can use rocket to overtake the leader or if you have shield and are in the middle of the pack. Use turbo after the 90 turn, after the wide bend or after the last pair of bends. Ideally you can overtake the leader after 3 or 4 laps.
Sol 2, I got it on the first try with Feisar, I was probably a little bit lucky, I got turbo in every lap.
Sebenco was one of the easier, but I love that track and I'm good on it. Sideshift every bend and hit speedpads on every. If you hit speed pad after the 4th, barrel roll on the big straight fall before the tunel. Tunel exit is ideal place for turbo.