Well, I've got four pallets in a one acre pond. One is right below the surface with rocks piled on it. One sank, and the other two roam all over depending on the wind.
I have 8" gravel in beds around the shallow end for SMB (someday..) with smaller gravel piled on top of the 8". I've noticed the spawning FHM hanging out about two feet apart all around the big 8" gravel. It's piled, so there's a lot of gaps. I guess this is where they are spawning, going to spawn. No FHM around the pallets. I'm a total rookie at all this, I'm just watching and learning.
I do know when the feeder goes off the FHM seem to materialize out of nowhere in rivers of minnows.

12 yr old pond, 1 ac, 15' deep.
RES, YP, GS, FHM (no longer), HBG (decreasing), SMB, and HSB (only two have been seen in 5 yrs)
I think that's about all I should put in my little pond.
Otter attack in 2023.