Wait. When you're blacklisting the AST GPU you have to switch to the AMD GPU as well.
Means, after you are hitting enter in Petitboot, you have to switch to the port where the AMD GPU is connected, immediately.
Otherwise, the OS don't recognize the AMD GPU.
If you still stay on the AST port after blacklisting this GPU, you will get exactly this picture. It is logical that the Display Manager cannot start then.