Do-able but more days better
Yes, you can get to Bandon from Seattle in one day. But you will have to get up early and drive a long day to do it. And there are so many things to see and do along the way that this will give you virtually no time to stop and explore along the way.
If you've never been on the Oregon Coast, you will be pleasantly surprised by it. There will be numerous outlooks you might want to stop and linger at, places where a walk on the beach is in order, interesting shops, fun touristy things (Tillamook Cheese Factory, Sea Lion Caves, etc.) and some great historical stuff (Fort Clatsop, for one). So, if you don't mind zipping by these things and just seeing the sights you can see from your car windows, by all means drive all the way to Bandon.
The two-lane, windy roads don't lend themselves to a speed run. Traffic in general isn't usually too heavy except on holiday weekends. However, it doesn't take much traffic to slow things down. Add in a few RVs along the way, and you can be assured of a slower drive. And, actually, this is no big deal if you just decide to sit back and enjoy it. But it can be quite annoying when you're trying to burn miles.
I can't tell you exactly how much time it takes to get to Bandon because I've never made that trip without numerous stops. And I've never done it without planning at least 2 days to get that far anyway.
I think you would have a much funner trip if you do the following: make this trip with no destinations in mind. I mean, hope in your car, drive to the coast, and just meander/explore/enjoy the sights along the way. When it's time to stop for the night, stop. The same the next day(s). And then, when it's time to go home, zip over to I-5 and go home.
You may only get as far as Newport, or whatever, yet you will have had a great time. Then, some other time, drive down I-5 and pick up the coastal drive where you left off and do the same. Eventually, you'll make it to Bandon and the Redwoods.
My other suggestion would be to take at least 5 days for this trip, preferably the minimum of a full week, and then you'll probably be able to see most of what you'd want to see and still have time to get home. (Getting home from the Redwood areas is basically one long day up I-5).
