There is an excellent "fan" book out on Hamilton that has all of the backstories to all of the songs and lyrics. In it, LMM talks about all of the musical influences that entered into his thinking when writing and re-writing the lyrics -- most of them being old-school rappers from the 80s/90s. His line, "only 19 but my mind is older" is a perfect example of that, taken from an old Mobb Deep song. There are a ton of similar examples. This is the guy that wrote "In the Heights," as well.
I respect your opinion, however I totally disagree with your assessment. The lyric-writing is breathtaking. There are many liberties taken in the storyline, however it's not meant to paint a 100% accurate picture of things, but rather present history in a way that you haven't experienced, before. I will also say that you can try to break this show down across so many levels, but at its most base level, it's art, and everything from the in your face numbers (Satisfied, Wait for It, Burn) to the huge ensemble pieces (Battle of Yorktown, Nonstop), to the tearjerkers (Dear Theodosia, It's Quiet Uptown), to the goofy bits with King George, to the freaking "RAP BATTLES!!" to the subtle choreography that was mentioned in an earlier post, and that is a constant throughout the entire show, to the music (which, if you find some video of the orchestra from underneath the stage, is awesome), to the fact that it's all done with people of color, is groundbreaking. I guess I was lucky to have listened to the soundtrack a number of times before seeing it, so I had a chance to work through everything ahead of time.
Sorry . . . I sound like a maniac on this. I've seen a lot of shows -- I'm all for Oklahoma, Hello Dolly, and the others -- it's all great -- Hamilton is truly unique.