Set automatic marker

While I feel like there's probably a solution on AEScripts.com, it might be just as quick to hold SHIFT while scrubbing the timeline, so that the current time indicator jumps to the next layer inpoint and then hit * on the numberpad.  I know it feels labour intensive, but I've just done a quick search and not found anything obvious

Ignore that last bit, I've just asked ChatGPT and it came up with this:

// Get the active composition
var comp = app.project.activeItem;

// Check if a composition is active
if (comp && comp instanceof CompItem) {
    // Loop through all layers in the composition
    for (var i = 1; i <= comp.numLayers; i++) {
        var layer = comp.layer(i);
        // Add a marker at the layer's inpoint
        var marker = new MarkerValue("Layer " + i + " Inpoint Marker");
        comp.markerProperty.setValueAtTime(layer.inPoint, marker);
    }
} else {
    // If no composition is active, show an error message
    alert("Please select a composition.");
}

One quick test and it seems to work.