HTTP code for timeout when server continues processing in the background

I stumbled upon a case where a request to an endpoint might take more than 60 seconds (let's say that's the timeout value), in which case the server sends a response and continues processing the request in the background. There are also cases where the same request would be processed before it times out and a successful response would be sent from the server to the client.

What would be the best HTTP code to use in those first case? I read HTTP server timeout. When should it be sent, which suggests 503 or 504, and HTTP status code for 'Loading', which mentions that the request can be deemed successful and return 200. But I'm not convinced by any of those suggestions more than the others yet.