Fishing Rod has good questions about overflow topics. Answers provide insight as to possibility of invader fish being present. Even if the pond is flushed heavily most of the FHM should still be present. There is a significant reason the FHM are not obviously abundant in a fishless pond after 18 months. Detective work should answer this question.

The pond being dry and reworked in 2012 indicates the likelihood of fish kills are minimized compared to a tree lined, overgrown pond that is 40+ years old. A 11ft deep MO pond will thermally stratify with depths of 7-11 ft being devoid of DO and then a summer turnover brings most of that anoxic water up into the top 6ft. This situation is deadly for fish.

How big were the 250 BG just added? 2"-3"? With ample food some of those fastest growing early maturing BG could spawn in August of 2023. Do your best to visit the neighbors that have ponds for a fishing opportunity to buy a few adult BG for a May 2023 spawning.