Thanks everyone. For anyone who is considering the same question, i.e. getting from Irvine (or Anaheim) up to Upland/Rancho Cucamonga area...

I ended up riding two of the routes I was looking at, in a big loop:

https://imgur.com/a/CPjpm

Of the two, I think the one that takes SART, Green River, Railroad, up Hamner all the way up, is better.

I didn't consider the route that takes Carbon Canyon Road, I might try that in the future.