The inside of the fruit looks pretty good. If there’s not enough acidity, it could be the variety. El Bumpo has excellent sugar acid balance. Most Cherimoyas are pretty sweet but the longer you let it hold on the tree, to a certain extent, the sweeter it will be. Also, proper fertilization will increase sweetness.
As your tree matures, you will get more fruit and the quality will increase. With more fruit, you can wait until the first fruit drops and observe the color of the fruit to tell when other fruit will soon drop. You want to pick the fruit before it drops otherwise the stem end will pull out and the opening will rot or get fungus.
Simon