I'm on T-Mobile, too. I rooted mine back when it was still in lollipop and I just recently updated it to Eliminator's Marshmallow ROM.
It wasn't difficult when I first rooted it, but the new methods look a little sketchy to me. I'm used to rooting Nexuses, though.
You'll probably lose everything. When you unlock the bootloader, the phone gets wiped. I can't recall if you can root this phone without unlocking the bootloader.