How do I edit the thumbnails in Slideshow to stack in rows instead of a carousel at the bottom?

Try adding to Design > Custom CSS

.sqs-gallery-design-strip .sqs-wrapper {
    width: 100% !important;
    left: 0 !important;
    display: flex;
    flex-wrap: wrap;
}
.sqs-block .sqs-gallery-thumbnails .sqs-gallery-design-strip-slide {
    width: 20% !important;
}
.sqs-gallery-design-strip {
    height: auto !important;
    overflow: visible !important;
}