You could do both in five days.
Hi and Welcome to the Great American Roadtrip Forum.
Computer mapping programs will no doubt route you via I-5 on a direct route, which is the fastest and shortest... but not exactly the most scenic. You could go via LV and Death Valley and still drive to Seattle.
US-395, from LA to Seattle via LV and Death Valley is 1400 miles, which would be three comfortable day's driving less than 500 miles. However, there are of course two provisos. Firstly it is assuming you will not see any winter weather which could see you off the road. The other is, I am assuming that you are not just coming off a long haul flight, in which case we would highly recommend you spend 24 hours in LA.
The positive is that Death Valley would be fantastic in December. The heat of summer is gone. It is such a spectacular landscape. 395 in CA is a very scenic drive. I have not driven it all the way to Seattle.
If this is your first visit, may I suggest a few tips which have always been helpful to me. Bring your automobile club membership with you - RACV, NRMA, etc. It will give you access to free maps and tourism information from the AAA. You could pick up good detailed maps of each State you plan to cover while in LA. I have also found a flag in the back window of the vehicle an advantage. Let them know you are not a local.
Lifey
Attending to Accommodation.
Do you have accommodation in Yosemite. That may be the first thing to attend to. Check the NP site for any accommodation still available. If it is all booked up for the date you want, check back frequently as people's plans change and cancellation do happen.
I am not familiar enough with the area to suggest a half way point, suffice to say, any good map will give you options.
Lifey