I have similar hair to the OP - fine/low-po/easily overmoisturized. My question is: are you sure you need a leave-in? I don't personally use one because I found them all to be too heavy and to weigh my hair down as you described, with flat roots.

I low-poo or co-wash, use my RO, and then work KCCC through my hair *in the shower*, then quickly rinse with cool water to distribute it. This has worked far better for me, in terms of softness and definition, than any leave-in. So maybe experiment with not using one, or using a lighter product that still adds some moisture and definition instead of a "real" leave-in?
I sort of blog
I'm mostly away doing field work through mid-May.

Hair: curlybob, fine, dense, low-po, big waves/ringlets (2C/3A)
Co-Wash: Nature's Gate Herbal Conditioner
RO: YTC Carrot, Aubrey Organics GPB
LI: BS Shea Soft Styling Cream (HG)
Styling: KCCC, Garnier P&C Gel
1xWeek: low-poo, gelatin PT or Joico K-Pak
Hair Likes: aloe, coconut oil, humidity, protein, long walks on the beach