By the time you can craft sugar cane auto farmers you can craft pumpkin auto farmers, which are more emerald efficient. Until then I would recommend a meat farm (pigs/sheep/cows) to level butchers, and sweet berries (which grow insanely fast and just need dirt and light) for emeralds.
Farmers are IMO much better for getting emeralds than librarians, because you can trade both pumpkins and watermelon (which the crafter block now makes less of a pain to store). In terms of auto farmers with chests for overflow storage, watermelon + pumpkin are much more emerald dense than paper too (6 pumpkin/4 melon per emerald vs 24 paper per emerald). Plus you can trade for golden carrots with maxed farmers, which are one of the best foods for hunger saturation and can be used to breed horses.
Pumpkins can also be used to craft jack-o-lanterns, which are convenient early in the game as lit blocks, and the seeds can go straight into a composter (which you’ll conveniently have right next to the farmer).
Also you need leather to craft books (unless you buy bookshelves and chop them down, which I find annoying), which brings us back to cows, and if you have cows you may as well have sheep and pigs, and a butcher.