Here is the routes I did as part of a larger tour to get from Spokane to Missoula several years ago except I added the direct route between Taft Jct and St. Regis using the Olympian Trail to the collection.
https://ridewithgps.com/collections/1852267?privacy_code=nwhUqKpTk6e1tPr1 We really enjoyed the sections between Spokane to Coeur d'Alene; Hamilton, ID, to Mullan (we took the interstate east of Mullan to Taft Jct instead of the trail as the trail was in rough condition that year) plus Wallace was a pretty cool town; and the Route of the Hiawatha (Taft Jct) to St. Regis via St. Joe River Rd. As a matter of fact, I am re-doing Routes ROTH and AIGC again this summer on my tour from Alaska to Mexico because I enjoyed it so much, assuming the bears don't eat me in Alaska
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The sections of I-90 we rode were perfectly fine. Wide shoulders and relatively low traffic made even my traffic-adverse wife say it was fine. As a reference, the AADT on I-90 is ~8k per day. That is probably total but even if 8k in one direction, that is not that much considering it is a 4-lane highway with full shoulder. Plus remember that traffic will run much heavier at night compared to a regular highway so the daytime numbers are slightly lessened. I would suggest you look up an 8k AADT road near your home that you ride and then compare it to the 4-lane with shoulder version.
Regarding your St Regis route. I personally would use my routes between St. Regis and Missoula as there are lots more services and the traffic was very reasonable overall.
During the tour I did the above routes, I continued on to Yellowstone via Bozeman (cool town). I did these routes:
MMRC-E:
https://ridewithgps.com/routes/42128462 RCPM-S:
https://ridewithgps.com/routes/40853016 (Scenic unpaved route along Rock Creek Rd.)
PMAM-E:
https://ridewithgps.com/routes/42128496 AMBM-E:
https://ridewithgps.com/routes/42128543 BMWM-E:
https://ridewithgps.com/routes/42128579 You would connect with the Lewis & Clark Route in Whitehall. As a side note, I prefer the Parks, Peaks, and Prairies Route over the L&C Route due to much nicer scenery, more services, etc. If you decide to ride the PPP, I would take the TransAm from Missoula to Yellowstone where you connect with the PPP. You can break off the PPP in a few places east of Rapid City and use this collection of routes
https://ridewithgps.com/collections/54989?privacy_code=3XmzOFv1x698fxdN to get to Ludington, MI, saving some miles. You would connect with the Lake Erie Connector near Ludington. Almost the entire route across Wisconsin is on rail trails.
I have ridden all the listed routes and have actual cue sheets and GPS data for most of them though the cue sheets and data might be off slightly in places if I updated a route after the tour.
Whatever you decide, I wish you a wonderful tour!
Tailwinds, John
P.S. I did not have truck issues on any of these routes.