I have used the API to create custom reports,
but it appears that there is no way to download the content of the report.
You can only GET the metadata of the report you uploaded.
Could anyone confirm that there really is no way to access the content of a report created via the API ? (Even if it was created via the API)
Per our docs, it is not possible to get actual data using the Report API; you can only retrieve report definitions. The API only allows you to create and publish reports to the Report Admin page where a merchant can then access the data.
I also do not believe there's a way currently to access the data in reports. The closest I can think of is using the ShopifyQL to replicate the queries used to generate the reports. To use ShopifyQL, you have to make requests using the Shopify App Bridge (previously known as Embedded App SDK). Check out this tutorial on using ShopifyQL.
I hope this information clears up any confusion and is useful for your purposes. If you have any additional questions about this, please let me know.