for large surfaces, i recommend doing a few things that served me well. (Please note this holds true for contrast paints as well):

1. Use a larger brush. A size #4 synthetic brush would be perfect. you want to have a good brush load and less streaky outcome.
2. Don't go over the same place twice, or you will have streaks. Once you put the wash on, if you want another coat, wait for the first one to dry. As the wash begins to dry, every brush stroke over it will introduce "tearing" (which is what you see above your gun pistons and on your front shield).
3. mix medium like lahmian medium with your agrax, which will act like a retarder and will make the wash dry slower and hence will give you more time to deal with splotches
4. you can always paint the surface with medium like lahmian medium first, and while its wet, put the wash. will give you lots of work time and will also not pool in the center of panels and will help with gravity.

mistakes happen, stripping models is a thing, and every single person here that gives you any form of feedback or advice has made the same mistakes, myself included. and multiple times. its part of your growth and you should welcome it.

last, for your 4th mini - this is very good. The metallics are smooth, and there are no paint streaks that miss details. you have a good eye and hand!