I am also frustrated with anon. For me they grow beautifully, have strong trunks, nice full canopies, and get lots of sun, but of the 4 anonna I have I've only had two fruits this year from my Geffner, zilch (2 sugar apple, one still small custard apple) from the others.
My two sugar apples are rather large between 8' and 12' tall (1 1/2" trunk in ground 2 years and 3+" trunk in ground 4 years respectively) and neither have even flowered... considering years ago I had a 5' tall sugar apple with a dozen fruit my first year in the ground I can only speculate that it's the trees not me or my soil.
My soil here is sandy loamy and I fertilize not so nearly often as you do maybe quarterly. I'm giving them 2 more seasons then the 2 sugar apples get the yank.
Yeah, I tried to gradually add small amounts because the leaves were chlorotic and I guess that was too much? I'm still quite the newbie with plants.Gotta learn someday right? lol

Wish you luck with your Annonas gnappi!