Hey Gianluca,

You can track the invoice thank page by doing some customization.
Is there a way to customize the thank you page URL so they can track it with analytics?
PayPlans Is integrated with GAnalytics to track the sales. You can find this app under backend Apps >> Click on new and find the list.

And a way to add some google/facebook code in the thank you page only to track it properly for remarketing?
You can find the invoice thank you page on below mentioned path, where you can put your code.
root/components/com_payplans/themes/wireframe/thanks/default/default.php

Let me know if you have any query.