One huge issue you need to prepare for is wildfires. This year and last were exceptionally mild fire seasons for CA. The year before you would not have been able to drive your itinerary.
Red Bluff and Redding are basically the same drive from Lassen. Since you're spending all day in Lassen there is no way you can make the drive over to Eureka but I would not recommend backtracking, just get to Redding or Red Bluff as those drives aren't bad. If the stop in Redding is only to sleep, you basically have your pick of all the major freeway motel chains. There is a new Holiday Inn Express that is quite good for the price. The only nice hotel in Redding is the Sheraton and that does have the advantage of being at basically the only Redding tourist attraction: the Sundial Bridge.
I don't know if you can fit it into your schedule but you may want to check out the
Shasta Caverns. It's a little detour but I think they're cool.
For Redwoods, I'd still recommend staying in one place. You'll spend more time packing up than you will save by switching. The National Park is great but you are absolutely correct that the state parks are just as good. There are also a number of private attractions too like Trees of Mystery. I think the SkyTrail is worth it but I'm sure there are those who would disagree.
Since you're still in the planning stage, just take a look at Shelter Cove. It's off the beaten path (maybe too far for your schedule) in what's called the Lost Coast. It's gorgeous and so few people ever make it out that way.
One piece of advice on dining is stay away from Black Bear Diner. They started in Mt. Shasta and have spread all over Northern California and OR. Years ago, it was ok...
You're definitely getting my bias against Portland so my apologies. It's like most things. When you live with it you tend to focus on the bad but when visiting you focus on the good. I really would look into Skamania. It's modeled after the National Parks Lodges and the views from the lobby/restaurant are just incredible. They also have a ropes course in the trees and if you book now you could probably get one of their treehouse rooms that always sell out.