Well, it's not so much that there's no point to CG if you use "non-CG" stylers. It's more that 'cones build up on your hair and need to be removed, and CO-washing doesn't get rid of 'cones. If you're planning on JUST CO-washing, you can see that this wouldn't work out well.
So, if you want to go totally CG, you'd need to get another leave in conditioner. Lots of people, including me, like using some of their regular rinse out conditioner as a leave in -- it works just as well.
If you really want to keep using your Garnier leave in, though, the amodimethicone will need to come out with something stronger than a CO-wash. You could use a sulfate shampoo once a week like you said, and that would work fine! Or, you can dilute the shampoo to make it milder. Or, still yet, you can use a low poo, a very gentle sulfate-free shampoo that will remove 'cones. It should have coco or cocamidopropyl betaine in it, because these surfacents will remove 'cones.
HTH! Good luck with returning to CG.