To hide previous / next arrows on Slider layout, you can use following JS code:

// triggers when gallery is ready
document.addEventListener('galleryReady:covetPics', function(e) {
 // prepare CSS styles to inject
 const customStyles = document.createElement( 'style' )
 customStyles.innerHTML = `
    .swiper-button-next, .swiper-button-prev { display: none }
 `
 // get Covet gallery elements where you want to inject CSS (for eg. covet-pics-widget, covet-pics-popup, covet-pics-upload)
 const widget = document.querySelector("covet-pics-widget");

 // now inject CSS code into shadow DOM
 widget.shadowRoot.appendChild(customStyles);
});