There is only one Interstate between Vancouver and Everett. To reach LaConner you will go west on on State Route 20 from the I-5 exit at Burlington. Highway 20 will most definitely be crowded on Tulip Festival weekends. It is a good road, but not an expressway.
Since you are only going as far as Everett on Sunday, it would be more scenic to go south on Hwy 20 (Whidbey Island) and take the Mukilteo Ferry across to Everett. However, it will be considerably slower than returning to I-5 to drive to Everett.
There are a couple of unclear/confusing points in your post.
"Which is my better option on Sunday I20 or I5 southbound at that time of year?..." I-5 is the major North-South highway through Washington State. I-20 is a major East-West highway through Texas. Did you mean SR-20?
In any event, the Tulip Festival may add additional traffic to already busy I-5, but not enough to point to it as a reason to avoid I-5. The normal traffic volumes, and somewhat boring nature of the drive along I-5 are enough to do that. :) For getting from Point A to Point B, I-5 will be generally best from a time in transit perspective.
From La Connor, you can travel on SR-20 down the length of Whidbey Island (where SR-20 morphs into SR-525), and take the ferry to Mukilteo and on to Everett. This is certainly a far more scenic route than I-5, but not a time savings. The wait for a ferry at Clinton can be considerable on Summer weekends. If you choose this route, check into the new Washington State Ferry reservation system as a means of making the time in transit more predictable.
I correct myself...
"If you choose this route, check into the new Washington State Ferry reservation system as a means of making the time in transit more predictable..."
Heh! Sez you, eh?