Create support for the long-running operations for the “calculate” (Excel) API in Microsoft Graph

Sometimes we have to find a solution even some proprietary API wont work.

I would say this is a valid option and my suggestion:

"Durable Functions is an extension of Azure Functions that lets you write stateful functions in a serverless compute environment. The extension lets you define stateful workflows by writing orchestrator functions and stateful entities by writing entity functions using the Azure Functions programming model. Behind the scenes, the extension manages state, checkpoints, and restarts for you, allowing you to focus on your business logic."

Create a long-running serverless workflow with Durable Functions - Learn | Microsoft Docs