Also that I must budget 5-6 weeks if I have any hope of making it all the way.
That isn't necessarily true, but I think it is a good idea to allow at least that. Notice that I said good idea, not absolute necessity. If your whole goal is a fast crossing, you are fit, you travel light or are car supported, and you train hard for it you could do it a good bit less. People have done RAAM in 8 days and the distance is about the same. Of course RAAM is a race not a tour.
My personal preferred approach is to allow a good bit more than I will actually take and then finish when I finish. Having an open ended schedule does complicate travel plans back home though especially if you will be flying, but much less so if you take the train, rent a car, end at home, or are picked up at the end. My last 10 day tour that meant allowing for 60 mile days and then averaging 80 mile days, but I am 58 and didn't specifically train for it (I was was trail running 4 times a week, but had almost no riding miles in for the year, so you could probably go much faster if that is your goal).
Just another couple points to consider in your planning...
The ST is usually done sometime between early Fall and late Spring when daylight is likely to be in shorter supply so be sure to take that into account.
Also places to stay are not typically spaced at a nice even distance so most days you will have to choose between two stopping places neither of which is the distance you want to average, this could mean that you might some days need to choose between riding 60 or 140 miles unless you stealth camp out in the middle of nowhere with no means of replenishing supplies.
I don't raise these points to discourage you from doing what you want, but so you can take them into account in your planning. And again I wish you success however you
measure it. I suggest that whatever you plan that you be sure it will be fun for you.