I wouldn't go too crazy with the rust effects. I found a couple of pictures to illustrate.
This first is a Sherman from the 3rd armored division taken in Germany in 1945. There's not a lot of rust on the tracks, more dirt and mud which is pretty typical for tanks. There's no rust to be seen on the tank itself.
This second is a 75mm Sherman in Italy. Again, no visible rust. There are a few reasons for this. First, tank crews generally took pretty good care of their tanks. You might find bent or broken fenders and thin sheet metal parts, but these usually weren't rusted. They would be painted over as part of routine maintenance. Rust on the tracks which would form quickly, many times even overnight, would be worn off by the drive and road wheels and any remaining would get dirt and mud covered within the first couple of hundred yards.
That's why I use a light touch with rust on the tank tracks but go fairly heavy with the weathering of mud and dirt. Just a few suggestions for you.
Your Sherman looks pretty good, just needs a bit more weathering. Remember, tanks didn't really move
over a battlefield, tanks tended to move
through the terrain.