Customize permission on Anonymous Link on SharePoint

Hi Xavier I would suggest using Dropbox.com or Box.com.  Both services provide very intuitive ways of adding external email addresses or sharing a folder publicly if you want to skip the login piece.  After that, Microsoft Flow can copy files from dropbox to SharePoint as shown here:

https://flow.microsoft.com/en-us/galleries/public/templates/4047185d35d041ca80c27e86f7bbbb34/copy-files-between-dropbox-and-sharepoint/

Afterwards you can then delete the file from Dropbox for instance.  See here:

https://docs.microsoft.com/en-us/connectors/dropbox/

Hope this helps and if you need help, my company may be able to help as we specialize in these types of things.  https://www.eswcompany.com/sharepoint-consulting-2/