Hi Chrise17,
I haven't seen anyone answer this, so, I want to at least respond to let you know that someone saw it.
It is my understanding that the 18-135 STM is a new lens and a new motor. It will probably be on par, if not a bit faster, than the USM lenses. I have to admit that I had, but got rid of the 55-255. I didn't like it, the build seemed cheap and the focus always seemed off. I ended up getting rid of it fairly soon.
I don't know anything about the Sigma 28-250. If you do get it, it may replace all of your lenses. However, I would hang on to the 17-85, it is a fantastic lens and may be very useful if you still want to do portraits or something similar.
Good luck,
Frank