Your stocking timeline will work depending...The FHM's will not last very long ( a season and a half). I believe you are feeding the HSB pellets, so it's not a big deal really. FHM's like to have a full season or so by themselves to "do their thing" and create a pond full of forage (free natural food source). You need to stock large enough Tilapia so that your growing HSB don't eat them.
Have you estimated how much chlorinated water was added to the pond? What percentage of city water to pond water did it calculate too? I ask to help understand how long the filter would last. Make sure the filter removes chlorine. I bet that would be tough thing to get out at a water flow that would suit your needs. IDK.
I would bet that 3 things may have contributed to your fish loss. Water temp change, reduction of dissolved oxygen, and the introduction of chlorine The chlorine being the biggest suspect (and likely the only reason) if you are using city water to top the pond off. Fish don't like it one bit. I am sure that they can stand a little, but do not know how much. The safe answer is to remove the chlorine before putting the water into the pond. Chlorine will leave the water given about 24 hours given the surface area to breath. Adding small amounts of chlorinated city water daily MIGHT be the way to attempt keeping the pond full without de-chlorinator. That would be an experiment risking the fish, however. This is still not healthy for the fish even if it does not kill them. Chlorine = Bad for fish.
Other options would be...
Dig a well and use well water,
Let the make-up water sit in open top vessels, like watering troughs, for a couple of days before releasing into the pond,
Using the troughs to de-chlorinate the city water before releasing it into the pond, or
A filter may be an option, but sounds expensive long term at the size you would need to condition evaporated volumes of water daily. It would be interesting to look into though.
My 2¢
