Best route to Bellingham, WA from Indianapolis, IN

I don't think it makes a big difference. I94 (and 90) is the shortest route between Chicago and Seattle. But on a 4-6 day trip a few hours driving time doesn't matter much.

The final pass over the Cascades will be as noticeable as the ones in Montana/Idaho. Not has high, but Seattle is at sea level, and the Columbia River crossing nearly so.

WA20 is a scenic way of bypassing the Seattle area, but is a higher pass.