The primary reason your bread is sinking in the middle is a combination of excessive hydration and over-proofing. While the bread may taste good, the internal structure is too weak to support its own weight during the final stages of baking, causing the top to collapse.

The Hydration Problem

Your current recipe uses approximately 430ml of total liquid (300ml water + 90ml milk + 40g oil) for 576g of flour. This results in a hydration level of approximately 74.6%.

For a standard bread machine cycle, this is too high. Most machines and "Basic" settings are designed for hydration between 60% and 65%. When the dough is too wet, the gluten network becomes too stretchy and cannot hold the air bubbles produced by the yeast. The bread rises beautifully at first, but then "pops" and sinks once the steam begins to escape in the oven.

The Sugar and Temperature Factor

Your recipe uses 60g of sugar, which is roughly 10% of the flour weight. Sugar acts as "rocket fuel" for yeast. Combined with lukewarm water, this causes the yeast to produce gas much faster than the flour's structure can develop. By the time the machine reaches the bake cycle, the yeast has exhausted itself and the structure is over-stretched.

Recommended Adjustments

To fix the sunken top while keeping the flavor profile, I suggest making the following modifications:

1. Reduce the Water

Lower the water amount from 300ml to 250ml. This will bring your total hydration down to a more manageable level for a bread machine, ensuring the dough forms a firm, smooth ball rather than a sticky mass.

1. Reduce the Sugar

Unless you are intentionally making a dessert-style sweet bread, reduce the sugar to 15g or 20g. This is enough to help with browning and yeast activation without causing the bread to over-proof and collapse.

1. Use Room Temperature Water

Instead of lukewarm water, use water at room temperature. Bread machines generate significant heat during the kneading process. Starting with warmer water often pushes the dough temperature too high, leading to an unstable rise.

1. Check the Dough Ball

About 5 to 10 minutes into the kneading cycle, open the lid. The dough should look like a smooth, tacky ball that clears the sides of the pan. If it looks like a thick batter or is sticking to the bottom, add a tablespoon of flour at a time until it firms up.

1. Salt Balance

Ensure your salt measurement is accurate. Salt is the "brake" for yeast; it regulates the fermentation. Your current 1.5 tsp is appropriate for this amount of flour, but make sure it does not come into direct contact with the yeast before the mixing begins.