You basically don't. You're adding a lot of complexity chasing after a very marginal (say, 5%†) fraction of the security application-layer crypto gets you.
You can get that fraction closer to 50% without a human in the loop by segregating your crypto code from the application in a virtual HSM, using something like a TLS client certificate to authenticate your application to the HSM, have humans in the loop for restarts/bringups of the HSM itself, and doing pretty aggressive monitoring on the request patterns between the app and the HSM.
The threat model here is that someone owns up your app server --- if that wasn't in your model, you wouldn't need application layer crypto. The idea is that owning up the app server will get an attacker enough access to make requests to the HSM, but not control of the HSM itself. If the HSM can detect abnormal volume of requests, you can use it as a circuit breaker to prevent bulk exfiltration of your secure data. You also get accountability, so that when your server is compromised you might know exactly what records were accessed.
Typically, the app server itself will handle all of the database operations, and the virtual HSM just provides a "seal" and "unseal" operation --- convert plaintext to ciphertext, convert ciphertext to plaintext.
† The 5% you're getting in the dumb online crypto scenario is that if you store the root crypto secrets in a file, an attacker can't necessarily recover it from an SQL injection attack --- but in reality the percentage is probably lower, since most of the time SQLI will equate to RCE.