I would pull all plants, clean the tank well, pull old dead growth, make sure substrate was clean, add clean water, and start over again. Then, once all that is done, and you have viable parts of the plants replanted, go slowly with ferts for the first few months. I'd hesitate to say that you may have 'overdosed' your plants, because I don't have as much experience as others here on PT. But that is what it looks like to me. If you don't do weekly 50% water change with the dosing you are doing, you can overdose your plants. I know you say you are doing that, but I think it is worth repeating. You can also make the tank sick from dead plant matter and hurt fish. The beauty of planted tanks is you can make mistakes, and fix them with a good cleaning and rescheduling of ferts. Good learning opportunity. Nothing most of us have encountered at one time or another when we first start keeping planted tanks!
P.S. Does your substrate also have ferts in it? If so, that could really take you over the edge with too much fertilization. You shouldn't have to add ferts to a new substrate that contains fertilizer.