Lastly, we will be driving there so we can cancel right up until the last minute but out of curiosity, how much terrain is open on average in early January?
This is one of those stats where the averages and percentiles tell a different story. The median percents open are about 10% higher than the averages, which get dragged down by the occasional severe early drought years like 2011-12 and 2013-14 that are only 10-15% open at New Years.
Squaw (steeps need a lot of coverage) and Heavenly (well leeward of the Sierra Crest) tend to be the laggards in reaching full operation. They average 2/3 (but median 3/4) open Christmas Week and average 78% (but median 88%) open mid-January.
You can add about 10% to all of the above percentages for areas that get more snow and/or are easier to cover, like Kirkwood, Sierra-at-Tahoe, Alpine Meadows and Sugar Bowl.
You will want to exercise that
cancel right up until the last minute
option in early January in about 20% of seasons.
Squaw is of course #1 in the region for steep terrain. Alpine, Kirkwood, Mt. Rose and Sugar Bowl also have their fair share. Mott/Killebrew at Heavenly are good too, but only about 50/50 to be open in early January.
Weekday crowds should be OK, but if it's a powder day/snowing traffic can still be a big problem. Get up extremely early and drive to the resort you're skiing for breakfast to get good parking and minimize the traffic issues. Stay away from Squaw/Alpine during storms as open terrain will be limited due to wind, visibility or avalanche control. Northstar is a good refuge below tree line on those days.