Question about PR best practices.

I work for an analytics company and often have to implement new ETL steps. The data transformations in these steps can be complex and the major changes are usually 500+ lines up to around 2000 (the last one had 765 lines just for schemas).

What's the best way to split up the changes into multiple PRs, bearing in mind that it isnt guaranteed that a file won't change as the change is built up?