Hopefully your little sick frog is still doing well!
Do you still have others in the original tank? I saw you said you removed the sphagnum moss and added more leaf litter. I've had success with and read that the more hiding places you give them, the braver they will become. Some frogs are just a bit shy and definitely appreciate the visual barrier that some good, big leaves supply. I've seen some tinctorius individuals do some serious climbing, so I'm sure they would appreciate some branches. I would look into getting some scraggly driftwood as it will help fill out the empty space that you have. You could always try to mount some broms on to it as well. Broad leafed plants like Philodendrons and Pothos will also help keep your humidity up and give the frogs places to climb on/hide as well as help fill out some of the tank. Not to mention, you can get some good types of these plants for relatively cheap.
Whenever you're feeding them, you can add a slice of banana to the tank to encourage the flies to stay in one spot, making it easier to keep an eye on how much they're actually eating. That's what I've done for froglets before and its very effective.
Hopefully everything works out and please keep us updated on your little one!