The costs of lambda or even DDB are IMMENSE. These only pay off for services that have a high return per request. I.e. if you get a lot of value out of lambda calls, sure, use them. But for anything high-frequency that earns you little to nothing on its own, forget about it.
Generally all your critical infrastructure should be Cloud independent. That narrows your choices largely to EC2, SQS, perhaps Kinesis, Rout53, and the like. And even there you should implement all your features with two clouds, i.e. Azure and AWS, just to be sure.
The good news is also the bad news. There are effectively only two options: Azure or AWS. Google Cloud is a joke. They arbitrary change their prices, terminate existing products, offer zero support. It's just like we have come to love Google. They just don't give a shit about customers. Google only cares about "architecture", i.e. how cool do I feel as engineer having built that service. Customer service is something that Google doesn't seem to understand. So think carefully if you want to buy into their "product". Google, literally, only develops products for their own benefit.