That is quite the property and you have your own switchback road to the lake, awesome. I hope things are balancing out like you hoped? I'll offer my advice but remember, it is free and you get what you pay for.
In creating a self sustaining eco-system for a man made "water feature", the less you do, the more you accomplish. Nature always finds a way, if you allow it.
I wouldn't worry about the snapping turtles but I disagree they do not eat healthy fish. They definitely eat fish, healthy ones included. However, they will find balance with your lake and if they grow too abundant, Great Blue Herons will start to nest on your land. Chances are, foxes will roam in and find the eggs, then you'll have a fox den.
The only thing I do, is twice a month I test water quality (pH, ammonia, nitrite,phosphate, etc), dissolved oxygen and a secchi disc. I log it into an Excel spreadsheet and watch for trends or anomalies.
Good luck, I'll be watching for more updates.
If a turtle can eat a healthy fish, that fish was too stupid to be in the pond anyway....a turtle catching a healthy fish is indeed possible, yet very very rare, and quite well documented