Here is a link to the ride report I did on the trip my son and I took to CO last summer. We hit Last Dollar road, Ophir, Cinnamon and Engineer passes.
The relevant info to your situation is that we both had limited off-road experience and big, heavy bikes. (Mine more so than his.) We had a blast. I didn't think any of what we rode was super hard, although there were some tricky parts. You really need to be comfortable with steep, rocky, downhill switchbacks. Easy, right?
Go ride some dirt on your GSA and see if you like it. Figure out if you can pick up your bike by yourself or not. I can pick up my Tenere, loaded or not. That's a big deal. I don't think I would ride dirt alone if I couldn't. (You've got time to hit the gym and work on your squats and deadlifts, lol.)
When you get to CO next summer it sounds like you have plenty of time to hit easy roads first and work your way up in difficulty as you feel comfortable. Even if you never get to Engineer, you won't run out of fun places to ride.
Oh and read all of EndertheX's CO ride reports. Lot's of great info for planning a CO trip!