If you’re okay with just visually hiding the posts, you could use Custom CSS to accomplish this.
Navigate to Design → Edit CSS. Paste the snippet below and hit Update CSS.
/* Show all posts when pagination is present, but only the first one when not. */
.container:not(:has(#post-nav)) .list-item:nth-child(n + 2) {
display: none;
}
The CSS above is tailored toward Kiko and your blog’s current settings. It might break if you change the theme in the future. Also, it uses the modern :has selector and won’t work in older web browsers.