I would say overall Hwy 20 would be a best option to Ukiah from I-5 at Williams and is a pretty good stress free route throughout but a little longer....mainly wide two lane with a few passing lanes in the hilly areas....dealing with Sacramento traffic during the daytime will be the only major obstacle.
Overall - taking Hwy 12 thru Fairfield to Sonoma can be busy, and from Sonoma to the outskirts of Santa Rosa before reaching at the 101, parts of that stretch is like a forest backroad that might be somewhat busy nowadays also, however it would be a shorter overall route...
Hwy 29 thru Napa and St. Helena would be real busy in the daytime...I wouldn't recommend Hwy 29 thru Calistoga and Lower Lake at the Hwy 29 at Hwy 53 Junction if one is towing something large due to the Stevenson Grade which is a curvy stretch, same for Hwy 175 after Middletown once you climb up to Cobb and thru Hobergs - I driven these roads too many times to count.
The Hwy 29 at Hwy 128 Junction at Calistoga thru Geyserville I have not driven throughout, however I have driven on parts of the 128 with all them petrified forest wooded back roads taking shortcuts to Santa Rosa....I would avoid the 128 at night if one is unfamiliar with the surrounding area due to constant directional changes at crossroad intersections, however that is another shorter route option and shouldn't be too bad overall.
The only minor issue you will encounter on Hwy 20 will be the stretch along the Clearlake shoreline with moderate to heavy daytime traffic just west of Hwy 53 intersection between Clearlake Oaks and Nice, although traffic should be still flowing well enough...
Glenhaven to Lucerne stretch is a potential speedtrap area patrolled back in the day with 35 to 45 mph speed limits, and prepare to get profiled by either the Lake County Sheriff or the CHP if one displays any out of state plates, especially Alaska plates thru this area.
Once one reaches past Upper Lake (westbound), Hwy 20 becomes more open road again all the way to the 101 and to Ukiah will be a breeze.
My Mom relocated in 4 different locations throughout that whole Clear Lake corridor between 1982 and 1991 which is why I am so familiar with that entire area upon my visits and staying there on a 5 month stint...
Her last residence was at one of the trailer parks along Hwy 20 between Clearlake Oaks and Glenhaven as I finally had the money to relocate her to live with me in Alaska during 1991.
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