Here are my succesfull first time cherimoya grafts. I did around a dozen grafts past september , all were cleft grafts and only two took. I left two leaves on each rootstock, directly below the graft point, to have the sap flowing.
The other two grafts are aproach grafts and this tipe of grafting i would highly reccomend, i set two such grafts and both were succesfull, i removed the grafted plants from the mother plant in around a month ,also were grafted in september. I left 4 leaves each plant and apparently the heal was good enouth and they survived, made a little growth since then also.
I have some more seedlings and will aproach graft them this spring with my two big cherimoya seedlings , one of which showed its first flower buds this winter.
I would recommend aproach grafting, as long as u can manage to keep the potted seedlings fixed stable in the crown of the tree and also prevent the soil in the pots from drying up.