Factory TRD Off Road and TRD Pro wheels are 60mm offset. 35mm will give you a bit of poke, but with 12.5" wide tires, I agree you'll probably have to chop the cab mount.
For comparison sake, I have 35.1" Duratrac 285/75R18s. These are skinny 35s. I'm running a 2" lift up front from the factory TRD Pro suspension. I only had to remove the small mud guards in front of the front wheels. That's it. No cab mount chop. This setup works great for me.
With 12.5s, you may also rub a bit on the sway bar, but maybe not with that offset? A big plus to that offset will be the extra clearance it'll give you for aftermarket UCAs like you're planning on doing.
Guys also report rubbing on the front skid plate with wider tires. Depends on the plate and the offset, I guess.