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Forgot to post update but in case it's useful, using the azure classic provider vs. the Azure Narive provider allowed me to create a ServicebusQueueEVentSubscription:

_, err = eventgridc.NewSystemTopicEventSubscription(ctx, "systemSubscript", &eventgridc.SystemTopicEventSubscriptionArgs{
		SystemTopic:               systemTopic.Name,
		ResourceGroupName:         resourceGroup.Name,
		ServiceBusQueueEndpointId: queue.ID(),
	})