Common pond snails seem to show up in every pond on their own. Ramshorn snails sometimes do, but they are not quite as preferred a food item. I think trying to establish grass shrimp would be a worthy effort. Fingernail clams would also be a good food source to attempt to establish. If you already have Asiatic clams in your area, they have been shown to increase RES growth rates. However, they do have some inherent risks.

Getting RES established in a mature pond such as yours will be a substantial challenge. You may have to rely almost solely on capturing large adults from other bodies of water and transferring them to your pond.