I am by no means one of the local experts, and this suggestion will not be agreed upon by all, but if my only goal was Trophy LMB and I had a pond your size, I would consider Gizzard Shad. They can ruin a BG population and overpopulate a small BOW quickly, but if you have a decent population of 5+ pound LMB you may be able to keep them relatively in check. They are more hardy and grow larger than threadfins and will reproduce very well in most pond settings. A lot of discussion on them here, do plenty of research before going that route.

Lake Chubsuckers may also be an option, but they are hard to source and will have trouble getting established in a pond with existing large LMB.