By default, <summary> inside <details> gets a display: list-item;, that’s where the lil triangle ::marker comes from. So it follows that if you change the display value, you will lose the ::marker.
BUT! This doesn’t seem to be the case on Safari. 🤦🏼♀️
::-webkit-details-marker is what you want.
summary::-webkit-details-marker {
display: none;
}