Self-plagiarism is an offense caused by violating the expectation of originality.
When you write a progress report or a final report, there is no expectation of originality of the text per se. Rather, you are expected to have done original work during each period, and the report is expected to provide a clear and comprehensive presentation of the work that was done during the reporting period.
Thus, it is entirely legitimate to include text from an earlier report into a final report that covers the period of the earlier report. Now, it is quite likely that the text should change to some degree to fit the new context. Whatever changes you might make are not about avoiding self-plagiarism, but about how your perspective changes when you're discussing the whole project in retrospect.
To drive the point home: my own practice when preparing a final report is to begin by concatenating all of the periodic progress reports, add a writeup of the final section, and then reorganize and rewrite as necessary to provide a comprehensive final document. If I were instead to write the whole thing from scratch, I would consider that an inappropriate waste of my sponsors' money and also likely to end up accidentally omitting important information contained in the periodic reports.