While it isn’t possible directly in Sparkle currently, this is a case where a tiny amount of code can help solve the issue.
You add an embed in the thank you page and turn off “activate after consent”, then place this code in it:
<script>
setTimeout(function () {
window.location.href = "/";
}, 5000);
</script>
This will redirect to the home page (the /), you can change that to the filename of the page you want the redirect to go to.
The 5000 is the number of seconds (times 1000), so replace the 5 with how many seconds you want to wait before the redirect.