My family used to go to Rifle Gap Reservoir every summer for camping, fishing, boating. I loved it then, but I haven’t been back there in years. It is popular so it may be crowded. I think all the CGs in the park take reservations, so it might be hard getting a site. I have on my “to visit” list Blacktail Creek CG, east of Yampa - it has good Mtn biking. It is a small CG (10 sites I think) and doesn’t take reservations, so you may have luck if you arrive Sun-Thur. I also want to visit Rifle Falls State Park - the hiking is supposed to be great. If you are interested in boondocking, there are a lot of dispersed camping sites along the drive between Yampa and Meeker. I keep track of dispersed camping info in Evernote and here are a couple links I have for the area.
http://www.twohappycampers.com/a-mid-week-getaway-to-the-flat-tops-wilderness/
More generally: https://www.uncovercolorado.com/scenic-drives/flat-tops-trail-byway/
Also, look up Browns Park National WL Refuge for amazing scenery and solitude - it’s far enough away that you won’t find the weekend crowds. I love the Gates of Lodore area, but I frankly don’t know much about camping there except from a raft! We have floated it several times and it is so beautiful. The Gates of Lodore CG will be full of rafters, though. That might be entertaining, but it won’t be quiet or peaceful!
All this talk makes me want to pack up and go! We are going up to Lake Granby next week, but it won’t be wild, remote and stunning like the areas you are going to visit. But we will be camping with friends with kids and will spend our days fishing, paddleboarding, windsurfing and having general family fun!
I don’t know if any of this is helpful, I think you know Colorado better than I do (and I’m a native!). Have a great trip!