@Steven Devooght My apologies for not getting back to you sooner. I was quickly swarmed with work after our meeting with the product group and reply back slipped my mind until now.
The product group verified that at this time it would be best for you to find a method that supports the Windows version you require. While there is an internal roadmap to update the Windows version, it will not align with your timeline. Due to the nature of how App Services is created on a shared infrastructure, there will likely never be the option to select your OS version on the multi-tenant version. This is a hard limitation again due to the architecture of the product.
Azure App Service does have the containers option where you can in theory bring a container with the version of Windows that you require. I would suggest looking into that if you want to stay within the Azure App Service product family.
If you have any further questions or concerns, please reply here so we can assist you further.