I did the trip from central Al. last year. We did it in 2 months but the next time we'll take much longer.
We saw some caravans along the way. They were gone way to early for us. Your daily itinerary may be your own, but when they leave you've got to go.
The only place we needed reservations was in Banff and Jasper because we were there over a long weekend. We usually called the day we were going to arrive unless we knew the cell service was awful, then we'd call the day prior.
I took some spare parts,(filters, belts, etc.) but no tire. I did blow a tire in Duluth going up, it was fixed that day. Further North tire availability may spotty if you have a bigger, special size for your rig. I didn't use any spare parts but I did need a couple parts I couldn't get till I got back home. That's the way it goes.
I would lean towards flat towing just because of the wear and tear on the dolly axle. The roads are generally good but some places not so much. I saw dollies being pulled with no known problems. I've done both and preferred flat towing.
Enjoy the trip!