Process large CSV file | forach limit

Hello there,

given is a CSV file that usually has between 500 and 1000 rows and consists of 31 columns.

I would like to change the file uploaded by a user, a certain value must always be set in one column and only under certain circumstances in another column.

Since there are no built-in functions for editing CSV files, I first tried to simply iterate through each field. This is of course bad in terms of performance, but this CSV processing is only very rarely necessary, so it would be ok. But this approach is not usable, because there is a limit - the number of statement execution seems to be limited to a certain value.

So I tried to iterate over the rows and manipulate the specific row in a different way. But here I also fail, I can't find a valid solution.

What I have tried:
Convert line to a list, change the values via the known index -> doesn't work, because list elements can't be changed.

Does anyone have an idea?