gallery Full Screen Auto Advance More than 10 Seconds?

Squarespace stores the settings in a data-props attribute, and it seems that if you can get to that and update it then the timer for the slideshow does appear to honour it. It's possible that there's a race-condition going on here, but I've tested it and it worked on my 7.1.

You'd need business/premium to do it because we're changing an attribute with javascript. put this in the page header code injection (Advanced in the pages Settings):

<script>
document.addEventListener("DOMContentLoaded",function () {
  const slide = document.querySelector(".gallery-fullscreen-slideshow");
  const att = slide.getAttribute("data-props");
  const attJSON = JSON.parse(att);
  attJSON["slideDurationMs"]=20000;
  slide.setAttribute("data-props",JSON.stringify(attJSON));
});
</script>

• waits for the document to load all its content
• finds the first fullscreen gallery slideshow in the page
• gets its data-props attribute and changes the sub-property of slide duration (in milliseconds, so 20seconds = 20000)
• update the attribute

Give it a try!