I think the ę at the end of sentence should not, or at least need not, be nasalized. B
There's a fair amount of variation in how people pronounced ę at the end of a word. Most speakers most of the time pronounce it exactly as e, but most speakers do have some kind of nasal sound at times especially in certain words and positions... As far as I know no one has carefully studied this.
My impression is that nasalization is a bit more common around breaks in speech (at the end of sentences) and when otherwise the first and third person verb forms would be the same (piszę vs pisze) and się when it's a bit emphasized. I have the idea that the feminine accusative ending is nasalized less often.
In more formal contexts nasalization is a lot more frequent. It's also common 'spelling pronunciations' where ę is pronounced... like ę in the middle of the word (pronouncing będę instead of the usual spoken pronunciation (bende)
Some years ago I translated a paper by a well-known Polish phonetician who said (paraphrasing) that never nasalizing final ę sounds too colloquial and always nasalizing it sounds artificially formal and the best style is to nasalize some and not nasalize others... (but with no info about which to nasalize).