Paint ?? A healthy pond will create a nice thin layer of algae on the inside within weeks, and after that, your healthy fish population will keep much of that under control picking at new growth..
As to the outside, that would depend of your need for cosmetics....................... Painting the inside would only seem right to me if you used and old fashion type "asphalt emulsion" ( a few coats) and a 2 week cure time ( like they use with concrete drinking water cisterns) .. A cheap and decades old proven coating.
Water health: More shade will help you maintain clearer water with less mechanical filtration / Edison. .. More turbulence will allow more fish per gallon of water. Plants of any type are generally good. Less food = less issues overall with water quality.
As to all the rest, only your effort and passion with tell the story as the years pass..
Enjoy.
Tony
