Thank you for the great trip report. The "Rainier Loop" is an interesting spin on the two longest trails at Mt. Rainier. You certainly get the best of both trails (with considerable effort.) You also did it fast! Wow.
I am fortunate to live near the mountain and have to done both the Northern Loop and the Wonderland Trail. Both are challenging and offer several side-trip options.
For those who decide to tackle the Wonderland Trail, I would strongly suggest setting up your trip to use the food drops offered by the NPS. You can mail your packages to ranger stations or if you prefer (and especially if fuel is involved) drop them off at selected stations along the trip. It's a significant amount of a driving, but it would provide you with a valuable perspective on the amount of vertical involved. Or for those who perfer to take s shower, you can stay at the Lodge at Longmire for a night, breaking up your trip depending upon your starting point.
The camp sites are all permit based, and I believe the permit lottery begins in March or April (maybe earlier). Anyhow, information can be found here:
On a purely personal note, I'd recommend staying at Summerland Camp (go as far into the camp as you can go, the best site is at the end). If you stay at Indian Bar, climb, climb, climb far away from the river (the camp nearest the privy). The temperature is at least 10 degrees warmer there in the morning and it makes a huge difference in my enthusiasm to leave the warmth of my sleeping bag. If you do stay in either Summerland and Indian Bar and it's raining, you sometimes can make use of the three-sided shelters...These are generally group reserved but often, the groups are small and there are plenty of bunks empty. This was especially welcome during a rain/snow storm that left everyone a bit weary after a long day of hiking.
The camp areas generally are NOT spectacular, at least compared to other memorable areas of the Cascades. But they generally are near water and like the article said, have a bear pole and composting toilet (including an a pretty fancy one at Summerland).
Also, regardless of which direction you tackle the Wonderland (clockwise or counter-clockwise), I would suggest traveling through Spray Park instead of the traditional route of the Wonderland up to Mowich Lake (which has a parking lot/awful camp ground) adjacent to it. You won't miss anything (the trail climb significantly in both cases) and the payoff of walking through Spray Park is completely worth it.
For those who prefer a little more off-trail adventure, you can obtain off trail permits. They do require you to camp I think at least a quarter of a mile off the trail and out of sight of anyone.
Finally, I think taking this trail slow has a lot of benefits. You can take the time to do the side hikes and if you do shorter distances, can get to camp earlier and take the better spots. Finally, depending where you start, staying at the lodge in Longmire or climbing an extra 1,000 feet to the Paradise Inn is a possibility for a respite.