Hopefully if you are a company that small, you haven't had time to develop multiple data warehouses. You can write up a script to query your single warehouse to get the necessary data. You won't create a unique response for each letter, except for filling in all the user's personal information. Instead, you'll write a letter like:

Here is a listing of everything you have a right to know about our company and processes under GDPR:

<huge info dump>

Here is all of the personal data we have about you:

<very long CSV file>

Ideally, the most time-consuming part of responding, after the first such letter, will be verifying the user's identity.