I would protest the point that you have to be as large as Amazon to produce your own database-as-a-service solution, especially if you are only building it for yourself. Throwing some automated tooling around Postgres and any arbitrary linux VM hypervisor API should get you a lot of the same value.

It doesn't take a ton of magic to allow a developer to spin up a pg cluster with a few clicks via some custom web tool. Reporting and operations are trickier to handle, but if you don't need to satisfy the same customer base as Amazon does (i.e. you are your only customer), you could probably pull this off at nearly any scale.

That said, I will certainly grant you the cloud vendor lock-in angle. It truly is one of the biggest ploys of this decade. My organization takes great pride in only using the most fundamental of cloud resources (compute instances, block storage, DNS, VPN, routing). If we had to evacuate AWS to on-prem or some other provider, it would not be a huge ordeal. For those who wound up with their entire business stuck firmly inside vendor-specific things like Lambdas, you have my deepest sympathies.