we've had the same experience. unless you want to go to the water park, 1 day is enough, reserve and ride will help you be able to ride some rides more often. We have also seen the park empty out a bit before close, but also they will cut off long lines before close (you can't get in line at 5:55 for some rides for a 6pm close).
Parking is not an issue at all.
we have stayed onsite and offsite.
We drive from norcal.
Biggest caveat for legoland is the age range is really 5-10 or 12 for best kid memories. Our 13 and 15 year old had fun but mostly in the "taking my 7 year old brother on rides" sense. There are not big thrilling rides for larger kids. Spring break we did universal, Knotts, Disney and Legoland, and Legoland was the least favorite for all but our 7 year old.