Hide out of stock variants

Store Design
6

You can try the CSS code below

First CSS code is to hide out of stock items

Second CSS code is to make the selected item more obvious.
Third CSS code will make your the gallery sticky on scroll until the end of the swatches.

#clickyboxes-option-colour li:has(> .unavailable) {
    display: none !important;
}

.product-form .selector-wrapper.selector-wrapper--swatch.selector-wrapper--swatch-image a.active {
    box-shadow: 0 0 0 4px #000 !important;
}

.product-gallery.half.column {
    position: sticky;
    top: 15px;
}