I drove the mountain loop last weekend. One suggestion is to head up to Boardman lake (which is closer to the Seattle side). There are a couple of decent boondocking sites up that road. Right now their is some road damage so it is keeping an otherwise pretty popular spot less trafficked. We had no problem getting a quigley e350 through the washout and saw a few lower clearance vehicles that (I think) went right through the mud. Lake Evan is a 2 minute hike from the TH, Boardman is < 1 mile. The swimming was superb a week ago. There is a boondock site a few hundred yards beyond the trailhead by following a track to the right. It is typical of dispersed camping around there some broken glass and lots of shotgun shells. Great night sky views, decent fire ring, and room to roam for the pups. The track was a little rocky, I had no issue getting down in in 2wd, the road is a little overgrown past the trailhead.
If you go to the Mt. Loop area I recommend taking a spur road to get off the Mt. Loop proper. It is dusty this time of year if you take one of the spots right on the road. There are some amazing spots to hang out at the river crossings, although they were totally full by friday mid-day last week. My favorite spot on the Darrington side is up White Chuck road. There is a track on the left maybe 1.5 miles from the highway that immediately T's to parallel White Chuck. I think the spot on the right is better, but both are good and relatively private. You can hike down to the river there.
Off Hwy 20 - head up Bacon creek road. There are a ton of great spots that are in the N.F. for boondocking. Google maps has it labeled as N.F. 1060. It is a few miles once you are out of the east side of Marblemount. IIRC there is a fantastic spot that can handle 4 vehicles about 4 miles up the road on the left. I've let the dog off leash there and she couldn't find trouble. There is a nice hike down to the creek.
There are a couple of limited spots near Diablo, but if they are full you don't really have anywhere else to camp for a long distance headed east.