========Problem solved =======
Thanks for your advice! I made sure to put id (or sku) before the numbers....
What I found out is that if you separate each script on a per page basis then it works!
So I put this in functions.php of the WooCommerce backend:
// price updates 2021 books
<script>
var element = document.getElementById("id440050").innerHTML = "€61,00";
var element = document.getElementById("id440039").innerHTML = "€42,00";
</script>
// price updates 2021 toys
<script>
var element = document.getElementById("id440051").innerHTML = "€91,00";
var element = document.getElementById("id440218").innerHTML = "€74,00";
</script>
And it just works!