So does anyone here know any notable examples of how spoken and written English has changed over the last 500 years? This will give me a jumping off point to create my own idiosyncrasies.
The easiest example I can give is to look at Shakespeare's plays - it's only 400 years-ish, but it ought to give you a good starting point. Just keep in mind that Shakespeare invented a lot of words in his plays, words that work so well we're still using them.
This is probably something most focused on by language scholars. How do the search terms 'language evolution', 'linguistic shift', and 'speech patterns' work out for you in Google Scholar? I'd also try 'Middle English'.
You want something wild, check out Seamus Heaney's translation of Beowulf. Old English on the left, Modern English on the right, and it's almost like they're in different alphabets, the text is so different. Further back than you're looking for, but rather interesting.