Hi @MO4 ,
You can do this with custom JavaScript. Add a custom javascript in web page / entity form. This is one of the way I think we can achieve it.
1. Get the form div id name or class name.
2. once you get the id add little jQuery or javascript to hide your submit button or the entire form. Little hint: https://www.w3schools.com/jquery/jquery_hide_show.asp on how to achieve this.
3 . Add a boolean variable and set the flag to true once SharePoint file is uploaded
4. Write a if condition and depends on the boolean flag to perform step 2.
This will give you some idea on how to achieve this.
Hope it helps.
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