Your experiment may have "failed" to some degree because it was in a new pond. Your pond is now significantly different as a mature 12 y.o. pond. You might try some modifications and have more success this time around because your pond is more fertile.

Also, do you have any other type of minnow in the pond after the FHM were presumably extirpated? Your pond looks pretty fertile to me in the pictures, so I suspect you had plenty of zooplankton and small aquatic insects to feed the YP and SMB as they moved from fry to fingerlings. However, as they moved from fingerlings to adults they sure could use an abundant minnow population to make that transition. Can you get any gambusia in your area? The ones in my creek grow very fast over the course of the summer and look like calorie-filled chunks to me.

Also, as Theo says, how is your spawning habitat? I was amazed at the larger size of the rocks used for bedding material by some of our members with good SMB recruitment. The guys with good YP recruitment frequently note the eggs draped on their spawning structures. Did you ever notice that, or work on that in your pond?

OTOH, there is a reason so many people stock LMB in their ponds. They are just easier to get good reproduction. If your family members are at the ages that there will be lots of fishing pressure over the next 5-6 years, maybe switch to a moderately heavy stocking of LMB and count on angling to keep their numbers correct. However, that will almost certainly require BG, which might not be a welcome addition for the recreational swimmers in your pond.

Good luck on your improvements for your Pond 2.0 version!