Best route

Last December we decided to go visit the grand kids in New England and we are near Seattle. We just watched the weather and made our decision on the day we left. We were in no hurry and planned to wait out anything that might come along and as it turned out, our trip over, our month in New England and our trip home, we saw one night of snow, period.
So if I were you, I'd google the fastest route and plan on doing it that way and have that alternate in the back of your mind, just in case
Saying that, for October, I wouldn't have a problem planning on taking I84. The only place that you might have issues with is out of Pendleton to Boise and even that in October would just be rain.

We went Austin to Portland a few years ago in Mar-May via I10 and I5, did the tourist thing on the way and stayed about a month in the northern Sacramento Valley mainly waiting for Spring. Then Portland back to TX via Yellowstone in May, so that took us over the Rockies. When we go back to Portland or any NW areas, I will use the I10-I5 route. I can make the Wx more predictable just by staying out of the elevations.

Thanks

Best way to miss bad weather is national weather forecast. You have several possible routes. I know how I would go, weather permitting I90, I82, I84,I15,US6,I70,US191,I40,I25,I10 to SA.

I5 to I10, but I bypass LA through Palmdale and San Bernadino