Well, it's a labor of love.
It's recommended to keep a pond (lake in your case) journal, so you can better track progress.
As the process will be ongoing, it may be beneficial to invest in some fish traps, nets, and seine nets. These tools can go a long way towards achieving your goals.
If you ever electrofish again, that is a great way to cull large amounts of fish.
You mentioned that the 'upstream' pond is frequently intentionally drained into your pond? I would try and stop that process if possible.
I'm trying to think of a way to do that. The owners of that 20 acre pond aren't very cooperative and generally asking them will get the opposite reaction. The spillway was damaged in my pond and I was trying to repair it I asked if they would be able to not block theirs for 1-2 days as the company repairs mine. Instead they opened it up.
