For what it's worth I think we spend a lot of time looking for the "perfect setup" when in reality there is a lot we can do to get by.
Since you already own the truck I'd keep it. Truck prices new and used are still insane right now, especially diesel, so it's not a great time to be a buyer. Buy the 5er you want, upgrade only if you're over your rear axle weights or if you've got the disposable income and really want to. Past that, take it out west and see how it handles. If you're doing these trips infrequently then you're probably going to be fine, just play it safe and follow the truckers in the right lane. You might lose a little time up and down hills, but you save a bunch of cash up front.
For what it's worth, I just came back from a NH to MT trip, and it was "fun" as my first time going through those kinds of mountains hooked to a trailer. I wouldn't want to do it in a gasser, but I feel more and confident I could easily.