Hollie, you will not be disappointed by sticking around for the Fall colors. We went through Glennallen on 4 Sep and just got back into Washington state yesterday. The colors all along the Richardson, Tok Cutoff, ALCAN, Taylor, Top of the World, and Cassiar Highways were unbelievable. The golds, greens, reds, purples, etc were super bright as most of this area had just had their first killing frost a day or two before we passed through. Most of the peaks had a light coating of termination dust which also added to the beauty! We did have 1/2 day of heavy snow along the Cassiar, but it was sticking to the road and made the scenery all that more impressive!
We had fall colors all the way to the bottom end of Fraser Canyon in BC.
We did almost the same trip in 2012 that started 10 days later with similar results. Leaving that late you will have freezing temps at night and possibly some snowy driving conditions. I'm sure it is nothing your DH can't handle! :-)