@Jona Thanks for reaching out. You are correct that while creating a subscription configuration for an Event Grid topic, the only supported methods are Event Hubs and Webhooks. Unfortunately, there is no built-in forwarding option for Service Bus. As mentioned in the below doc Event grid namespace will support all destinations in upcoming releases but not sure about ETA.
Reference: https://zcusa.951200.xyz/en-us/azure/event-grid/concepts-event-grid-namespaces
please do let me know incase of further queries, I would be happy to assist you.
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