Sarah -
Not sure of your schedule - sounds ike eastbound in early June?
I've done the North Cascades a bunch of times. First time back in 1989.
(I suspect that I'm just a few years older than you are.)
Eastbound usually in late May or June; westbound in August or Sept.
Eastbound can be a challenge because it can still be pretty wet & cold.
With more than a little snow remaining up at Washington Pass some years.
I remember, one year, a WSDOT road worker giving me some coffee
up near Rainy Pass because I was pretty cold and wet.
Westbound in lat summer there are often fire closures.
And even if the road is open, smoke from regional fires can be pretty bad.
Either direction camping is a problem because of the distance
between Colonial Creek and Mazama.
But Lone Fir and Klipchuck campgrounds are closer.
https://www.fs.usda.gov/detail/okawen/alerts-notices/?cid=stelprdb5308308Some sites in Klipchuck were snow-free 5-18, most should be by now.
Lone Fir was snowed in, but shoulf be partially snow-free.
Water is still off - not sure when it will be on or when you will be there.
If you leave early from Colonial Creek,
you should be able to make one of the east slope campgrounds.
Heck, once you top Washington Pass, it's a screamer downhill.
One of the biggest challenges of an eastbound Northern Tier
is hitting the Cascades passes by day 3. No question about it.
PS - Yuo do realize there are TWO passes?
Eastbound - First Rainy Pass, then a short downhill,
then more uphill to the higher Washington Pass.