If he is just killing off the weeds your fishing is not done. Fish will adapt very quickly. It might be 2 or 3 days but they still have to eat, and will eat. The weeds will not disappear instantly, they will die and then slowly sink to the bottom and decompose like you are experiencing. Your fish will go into a funk for a little and then change their habits, they probably won’t hit on the presentations you were using when the weeds were healthy. You are essentially doing just what you said, fall fishing. Think about september fishing and go at it.
A couple things I would try. First, if it is calm I would break out the topwaters. The fish should be able to see them from a long ways off now since the zinc will have killed off any algae bloom in the water as well as settled some of the debris suspended in the water. Number two would be a hard swimbait. Something in the 100-110 mm size. Swim it at a steady pace and if you hit weeds, drop the rod tip a foot and do a quick jerk. The weeds are dead now so they should rip fairly easy. Number three would be a spinnerbait, especially if the water has any dye or just murky look to it. They used to treat a pond I fished and it would turn blue, a white spinnerbait shined in that condition.
Look for any structure you can find, even just a lone stick poking up in the water will hold fish. They were used to hiding in weeds, they want to look for cover. If you can find a brush pile or laydown I would target that for sure.