Ask HN: Any advice on extra database security?

I am currently working on a DaaS API project. The company I work for has a customer who pays to query data. Because data is the company's most valuable asset, leadership is very concerned about its security. It is the first project in DaaS format, usually data is delivered within our application / UI. We are creating a separate database with only the data set for that particular client, which will be encrypted and unlabeled. This data will be available in a unique environment with a single API for this purpose only. The API has time limit on queries. If any of you have had experience in a similar scenario, would you like some advice or ideas to help me? What would be an outline of your solution for extra security in this case? Thank you all in advance!