US101 is good highway all the way down the coast. Many consider the stretch along the Oregon coast the best there is.
You will save some time by crossing over to I5 and taking that down to Grants Pass, but that would mean missing the Oregon coast.
US199 is the easiest route from I5 to the north end of the California redwoods. CA20 is the easiest way back to I5 at the south end.
I don't think it is worth while trying to see both the Redwoods and Lassen, especially if you don't like winding mountain roads. It would be easier to see some of the Washington (and Oregon) volcanoes, for example Rainier and St Helens.