...Why the last "ę" isn't everytime pronounced like the classical english "eom", but it's the obvious "e" letter?.
Short answer: Why should it be pronounced like the "classical english 'eom'" (whatever that is) Polish is a separate language and evolves in its own way.
At present most people most of the time pronounced word final -ę exactly like -e
like final -e but followed by a short nasal u sound (like final -eł but with the ł nasalizaed: this is mostly restricted to się (when sort of stressed) and maybe dziękuję, doing this in everyday normal speech very much makes you sound like a retired classics professor
e with a bit of nasalization at the end (a bit like French -in but not as strong) what many people do occasionally
like -em (considered sub-standard)
most style guides (for Poles) say using one pronunciation all the time is not good, never nasalizing final -ę sounds too informal and nasalizing all of them sounds far too formal. the recommendation is to pronounce some like -ę and others like -ę (which words are pronounced which way is left to the discretion of the speaker).
IME (when asking people) some cases of word final -ę are more likely to be pronounced like -ę than others, maybe especially when otherwise the third person form would sound the same (piszę vs pisze for example)
pronounce that particular sound as almost an 'uh-', or schwa-sound!
I think maybe that's Silesian? I remember a show with Silesian characters who pronounced both final -ę and final -em (as a first person marker) something like uh, I don't know if there's an accepted way to write that...