It depends on the load. You need to make your own calculations, for some sparse loads lambda gets the bill down up to 90%, for a load that is stable over time (like the 10 million request OP has), it makes sense to go with elastic bean or kubernetes.

You need to choose based on your load. Also, I would like to see OP slim down his app, so every lambda only has the code it needs to run and also has a permanent DB connection at least (lambda's good practices)