What pattern can you advice to solve a task?

I have an assignment and I would like to figure out which pattern is better to utilize.

The task is pretty straightforward:

Develop an application that consumes various text files which differ from customer to customer, parses them one-by-one and persists into the database in a unified form. The application is going to be used by numerous customers.

For example, one .csv file can contain the following payload:

day,time,year,name,surname
01,12:00,2019,Andrey,Arshavin
... 

Another file can look like:

day,time,year,fullName,country
01,12:00,2011,Cristiano Ronaldo,Portugal
...

And the database table has the following columns:

date(Timestamp),firstName(String),secondName(String)

The goal of the project is to provide a solution with a clear design.

The first idea that I have is to create different DTOs for different customers and one Entity class. On each .csv read, make a List<DTO> by using a factory method and after convert it to the List<Entity> and persist.

But how can I make it more extendable and maintainable? I think the spaghetti code is not a good idea for each new customer I will have to add

"if that customer then return such new object"