I doubt that potting mix with fertilizer matters much, I use potting mix like that all the time and the fertilizer washes out pretty quickly. No need to add extra fertilizer though.
I suspect that given the pot size/root mass that there is very little actual soil in the pot that can serve to retain water. So I suspect that watering weekly while seeming like a lot actually does not provide the plant with that much actual water. The vestigial leaves drying out and changing color combined with the wrinkling suggests that - though the color changing on the leaves could be due to a change in sunlight exposure, but if this plant is kept inside it hard to really think that would be the main cause.
I would find a slightly larger pot (not too large) so the roots can be encased in soil a little better, I think soil needs to be a little more water retentive that the current pot allows for and then only water when the pot is almost to completely dry. Then stop messing with it. The roots like fine and healthy so the less you disturb them the better.
Aside from that, the narrow new growth suggests the plant is not getting enough sun by far. I grow this plant's plant big brother (it is about 7ft tall) in half days (about 7 hours) of full sun in the desert here. It is showing some sun stress, but generally is taking the sun quite well (I water thoroughly once a week at the moment), so once it is back to looking plump and less wrinkled (some of the leaves will shrivel up and be lost from the plant - that is normal they tend to be only a feature on the newest growth on this plant), you should see about giving it some more direct sunlight especially if it is behind glass.
