I second David’s suggestion of Diamond Lake; I’ve camped there only with CycleOregon, but it’s a wonderful spot.
I’ve also Cycle-camped at the big state park campsites along the coast, such as the one at South Beach in Newport, and the Jesse Honyman Park in Florence. I was really impressed by how well kept they were, and how attractive, given their popularity. They may not be the best places in a pandemic era, though — and right now they are closed to camping.
Silver Falls state park is also beautiful; you may have ridden there on a Brevet. It’s developed, with a concession serving food and drinks, but has beautiful scenery and hikes.
At the other end of the spectrum, I’ve camped at a forest service campground near Oakridge. Just the other end of the spectrum in terms of facilities — water from a hand pump, vault toilets, and a (very cold) river to wash in! This would be a better place to avoid crowds, and there is good riding in the Oakridge area, as you probably know.
All closed right now, but Oregon is slowly opening, outside of Portland.
Andrew Black