how do I write colloquial speech, without jarring too-modern colloquialisms?
Fictionalize it. Just avoid the clichés that people would recognize; catch-phrases or gestures or accents, give them a twist and present your own.
Even then, the general rule in fiction (written or film) is to NOT be too consistent with accented speech, but spice phrases with it; or pick something that doesn't have too much impact on the reader understanding the speech.
Or skip the accents altogether: If you need a poor farmer, get into the head of one. What do they think about? What do they worry about? What do they talk about with each other? Do they think they're poor? Make their allusions and metaphors fit their trade; e.g. to describe something nearly impossible,
"Can't break that ground with ten ox pulling!"
An actual phrase from farming is "Don't eat your seed corn," meaning have the willpower to not sacrifice your future for the present satisfaction of eating. How else could you express that sentiment? Maybe, "better to eat a hen than to eat the last rooster."
what about the uneducated farmer in the same setting? Particular mispronunciations and grammatical errors..
Do not assume the lower classes are stupid.
There is a difference between being uneducated and being stupid. Most people are in lower classes due to birth circumstances. Both wealth and poverty are heritable; if your parents are poor they cannot afford your education, nutrition or health care, and you are forced into a life of early labor instead of more intellectual pursuits.
Poverty begets poverty; wealth begets wealth. In both cases, those cycles can be broken by exceptional intellect or talent, in the first case exceptionally high intellect or talent (although poor nutrition, health care and education may greatly reduce the odds of this occurring); and in the second case by exceptionally poor intellect or talent that squanders wealth to the point of losing its advantages for their descendants.
Do not include grammatical errors and mispronunciations to create colloquial speech. Almost anything you do in that regard will be considered racism or bigotry against a class, and alienate readers, and therefore agents and publishers that have plenty of other material to consider that doesn't stray into racism or bigotry.
You can use simple grammar, or a twisted but consistent grammar as some languages do, relative to English. "Cruel, he is. Believe you me," is not bad grammar, it is a different rule of grammar for the line "He is cruel. [you should] Believe me." The speaker may well consider the latter phrasing bad grammar; the "you" is missing from the second sentence. So there is an example of colloquial grammar; no assumed subjects. Or try the rule "no pronouns", or "nouns before adjectives", or other cultural modifications; e.g. men only refer to females by their name, never a pronoun.
Be consistent in the rules of grammar and pronunciation each character follows; don't try to indicate stupidity by language errors. Stupidity is conveyed through language by simplicity of sentences and concepts and misconceptions or superstitions; e.g. believing counter-factual claims. A lack of intellect has consequences in how much they can understand; both on the individual word level, and in terms of complex sentences or complex ideas. People of low intellect will reject the complex for a lack of understanding. Because they don't understand things well, they will cling to dogma and comfortable things they "know" are true (whether true or not) because they were told they were true by their parents and elders and community.
Intelligence is not marked by vocabulary (which is just memorization, and a good memory does not imply high intelligence); it is marked by insight and quick and accurate understanding of both what is being said and what its implications are. We consider Sherlock Holmes a high intellect not because of his vocabulary but because he sees clues where we see none, and puts them together into a coherent and true picture in ways we cannot. More often than not, in plain language: The clue is, "The dog that didn't bark." (Why didn't the dog bark? Because the dog knew the killer well, and did not regard him as a threat, and that narrows the list of suspects to one.) No fancy language, just a clever observation most of us would overlook.
The bottom line is do the work of a fiction writer, use your imagination to fictionalize the colloquial speech patterns you need, and in doing that you can avoid anything you recognize from real life. Clearly that is what was done with Yoda in Star Wars, and various curses and exclamations in Star Trek and hundreds of other fictional cultures and societies.
You don't have to be complex, or invent Klingon or some other language. Just a few rules (of grammar or culture) can transform a language. And mine the things and tasks these characters are occupied with all day for the metaphors and similes they would use; e.g. make them different for a soldier, a baker, a farmer or a seamstress.