If you decide to go back to I5 and up to Williams here are some directions to get you over to 101. I took 299 once years ago. Some say it is fine but I will not do it again. That was 20 yrs ago so I suspect it was improved a lot. The Clear Lake route is fine. If you really want to enjoy the beauty of the area take 20 straight to 101. It is a bit slow and tight but easy enough and you can see Clear Lake. I have traveled this route many times in our 40 footer with a toad. We now live in Florida so won't be back that way for some time.
This question comes up frequently so I saved this. Just reverse the route when going West to east.
Take Highway 20 West out of Williams. 37.1 miles. Williams is at I5 and Highway 20.
Watch for Highway 53 as you drop down a hill after a fairly long climb. Easy climb and multiple lanes.
Turn what would be South onto 53. You will go about 8 miles. Look for the sign to Highway 29 which leads to Kelseyville. This is a nice road and will become freeway in a few miles. Eventually you are back to two lane for a short distance and will come to a stop sign. You are back to Highway 20 again. This leg is about 30 miles. Turn left and enjoy the beautiful drive right to 101
This route will bypass the two lane twisting and winding on the North side of Clear lake. If you want a good view of the lake go ahead and stay on 20 all the way to 101.