[Glis Moriarty]

Like the title says, are there any good resources for the best way to write dialog for this genre? I don't want it to sound elizabethan, with lots of thees and thous, but I don't want it to sound modern either.

There's genre dialogue, some of it written by writers who have no idea except vague memories of old films, and some written by writers who do have an idea but are working to make it appeal to readers who don't know, but need it to feel right. And I don't know of resources for that except reading the most acclaimed books in the genre. Some genre writers have a long academic record of research in the periods they write about.

Then there's real dialogue and language. There are resources for that - collections of letters and diaries; and mustn't forget accounts of trials. Many of them are surprisingly easy to read. The Paston letters are especially famous. The problem with using this language is that the readers you are aiming at may not like the real thing. But it might help get you into the right mood. There are also examples of dialogue in mediaeval plays; few early records but quite a lot for the late period and Elizabethan.

 

[Glis Moriarty]

the flowery, formal way of speech we associate with older times, even up to the 19th century, was a marker of the educated upper classes. The lower classes spoke to each other much more casually.

All classes appear to have been pretty earthy for most of the period, except in formal situations. Clerks tended to dress it up prettily when they recorded it or took dictation.

Also worth remembering that English didn't start settling down until the end of the Middle Ages. French and Anglo-Norman were important after 1066, probably in decline after a few hundred years but remained important to the end. Latin was important throughout, but only conversationally important (probably) between important churchmen or as a lingua franca in people who shared no other language. At the beginning there was Danish (maybe some Norse) and Anglo-Saxon and the latter continued as the main language in some areas of England until the end of the period. Brythonic languages continued to be spoken in the west throughout. And that's just England and Wales.
But from the point of view of writing a book, most readers won't know that and it's best avoided unless you want to make a feature of it.

 

[Glis Moriarty]

A formalistic character may use thees and thous

Thees and thous weren't formal, just normal everyday speech across the population