Dear Julien Van De Casteele,
Good day. Thank you for posting to Microsoft Community.
Based on your description, you want to have a setting that only allows users to edit files that they upload.
You can't satisfy your requirement in the document library, but you can do so by editing permissions on a per-file. here is the step for you reference:
- Navigate to the SharePoint site and Document Library.
- Right-click the file > Manage access > click on the three dots in the top right corner of the pop-up window > Advance settings > Stop Inheriting Permissions.
- Remove all users’ permission, and then select Grant Permission. Enter the file owner > Share.
If you want other users to be able to read this document, you can grant them permission one-by-one.
You need to open the Permission level page and select the user > Edit user Permissions.
Also, I recommend you try posting in Microsoft Q&A Community and check if any PowerShell or customized solution could meet your requirement. Members and engineers there are proficient in the knowledge of SharePoint customization and development queries. For you reference: SharePoint - Microsoft Q&A
Thank you for your time and patience.
Sincerely,
Claire | Microsoft Community Moderator