I am NOT an expert, but have been following the history of many ponds on this site for a long time.
If you want a pond that most closely aligns with your goals, then the best solution is to drain the pond (and poison if needed) and start with a blank slate. You can then at least control your starting inputs and when you start to observe a problem, it will be relatively easy for the experts here to help you determine the cause.
IMO, that is MUCH easier in terms of money and your labor than trying to "fix" a pond with some unknowable parameters.
However, that solution is certainly very anti-naturalist!
The "start over" solution will kill all of the fish, all of the tadpoles, all of the aquatic insects, etc. The turtles, adult frogs, adult insects, etc. will still be around to re-populate the pond after it re-fills.
If you were really worried about those critters, then you could drain the pond to a tiny puddle, and then seine out the creatures to be saved and then move them to another body of water.
I deliberately wrote this post as a "stark choice". I wanted you to make a clear-eyed decision BEFORE you put in a lot of effort on your new pond.
Congrats on your new pond. I hope you and your family get lots of future enjoyment!
