Maybe I am a little biased, but once you define what SQL interface you want to provide to your users (so the schema of the tables) you have basically decoupled

Maybe I am a little biased, but once you define what SQL interface you want to provide to your users (so the schema of the tables) you have basically decoupled your internal data structure from the view of the client. At this point the only thing you need to do is to update the user facing database whenever necessary and keep it consistent with your view of the world. Yes, you have a new problem since you had to maintain the costumer view consistent with your view, but it may be a price worth paying. The way I would test this implementation is with RediSQL (https://RediSQL.com https://RediSQL.com) that I wrote (so I am definitely biased) but it may be worthed to test it.