OneC makes my hair droopy and stringy after a couple uses as well.
I do like NoPoo but regular use of that one zaps the little bit of volume I have.
If you wanna stick with the Deva products, I'd suggest picking up some LowPoo to cut down on the OneC build-up.
I'd definitely try an ACV rinse. Whenever my hair gets droopy, that pretty much perks it right up.
Overall, I'd probably NoPoo, ACV wash/rinse, then use a light co. Then just do NoPoo and the lighter co for a week or two and pick up a sulfate-free 'poo to use as needed (once a week or so, maybe). If you're still feeling droopy, you could repeat the ACV rinse in a week.
I would save the OneC for when you need deeper conditioning or to use on just the ends.
Have you been using the OneC as a leave in as well? If so, you might want to use a lighter leave-in instead. I found OneC to be way too heavy to leave in my hair.
Hope you are able to get some spring back soon!