OP-your Mustangs mane and forelock are much like my guys. SUPER thick. I have talked to a couple of major reining people about how to keep it. Those horses always have awesome manes. They actually pull it to trim it and get it to lay down on one side, and they trim it with scissors, just a little, which actually makes it grow better. After 35+ yrs of h/j and the mindset of never ever cut it....it was hard for me to do, but I will tell you-it did help.
In winter and spring muddiness I braid his tail up with strips of sheet in it, tie it up and wrap BELOW THE BONE with vetwrap. It stays on for 8 wks or so, until the spring muds are about gone. Really helps grow the tail.
I tried mane bags/braiding the mane last winter, and honestly, he was uncomfortable and rubbing, so I took it out. I was afraid he would catch a braid and pull out a huge hunk of mane, at which point I probably would have cried! Lol. He was better with nothing. I do not brush it all the time-only every couple of weeks, since it breaks the hairs, I think.
That is my .02.......Oh and for me-MTG did not work, and it smells AWFUL!