For your pico tank setup... since there is limited space and low-tech approach, just use moss for the mesh background, and moss on mesh for the foreground.
For small rooted plant can try cryptocoryne parva, it could work as a mid-ground plant in your tank size.
Btw, your current fish bio-load is too much for such a small tank (especially with low light slow growth plants which can't take up enough nutrients), remove the fishes (or reduce to just 1-2) and just have shrimps or snails in the tank to keep it in balance.
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