This year Spring break is March 4 - 20th. You will miss the largest crowds here this weekend. The crowds are pretty tame compared to 15-20 years ago.
The 45,000 number is optimistic at best, 25k is more realistic, when spread over 17 days is manageable.
The only big issue will be crossing the border both ways Fri-Mon during those 3 weekends it falls on.
Expect a wait when crossing at peak times. Look on-line for border waits.