The advantage of Serverless to me seems that it already forces you in a somewhat sane design and separation of concerns so all work out into the lambda functions should be easily translatable into a different architecture.

Yes, very much this. A lot of the effort was extremely in-depth planning for scalability with respect to both unbounded traffic growth/adoption and expanding the team. In fact I coded an initial prototype in about a month that could have run quite happily on a single instance and subsequently broken out into a typical LB/Autoscaling group/DB architecture without much trouble. Now we have about 7 core microservices which are independenty scalable and deployable, many of which we anticipate handing off to dedicated teams as we expand and hire.

Very smart. Good point.