No, it's frankly not possible to deploy Postgres at scale without a third party (or hand-rolled) HA/scaling solution. I worked on an application that hand-rolled one after considering several products.
I have deployed Postgres and MongoDB at petabyte scale. I am happy to answer questions from others who are intellectually curious about differences between the two that would matter to users who have to spend serious amounts of money on their databases.
I have used Garage for a long time. It's great, but the AWS sigv4 protocol for accessing it is just frustrating. Why can't I just send my API key as a header? I don't need the full AWS SDK to get and put files, and the AWS sigv4 is a ton of extra complexity to add to my projects. I don't care about the "security benefits" of AWS sigv4. I hope the authors consider a different authentication scheme so I can recommend Garage more readily.