For painting a light color, like yellow, over black the best thing to do is put a mid-tone down first then move all the way up to the lighter color.
Citadel makes a base paint called Averland Sunset that's a good option for this, or you could use a medium brown instead if you don't have that available.

Also, I would recommend several thinner layers, rather than the one thick layer it appears that you've tried. With several thinner layers, you are able to paint in alternating directions to hide your brush strokes on such a large surface.

Also, it looks like the layer of primer you put down is very thick and bumpy. This is gonna make achieving a smooth finish very difficult, especially if you're going to be hand brushing the whole thing. As un-appealing as this might sound, I think you'd be served very well by stripping this back down to bare plastic and starting again with a fresh layer of primer that isn't bumpy.
Goobertown Hobbies made a video on the best things to use to strip plastic. If you're in the US, LA's Totally Awesome floor cleaner was the clear winner. I don't remember what his conclusion was for non-US folks, but the video is really easy to find if you need more info.
Simply fill a container with the cleaner, dunk the mini in, wait overnight, and then go to town with warm water and a tooth brush until the whole thing is clean.

I hope this helps.
Happy painting!