Well, let's look at nutritional density from a nut, since they're among the most substantial of the "acceptable" Kypiq foodstuffs:
Per https://en.wikipedia.org/wiki/Almond it appears that 1 Acre of almond cultivation produces roughly 2250 lbs (1020 kg) of shelled almonds/year. From the same source, 100g of almonds accounts for 579 kcal. to give an adult male Kypiq a generous ration of 200g of almonds daily leads to a single acre feeding 14 men/year. (Only accounting for raw calorie count here)
Meanwhile, per the numbers at https://chroniclesofelyria.com/forum/topic/19930/size-of-elyria a Duchy has over 100,000 acres of available land, to a population of less than 1400.
Now, this is somewhat ideal circumstances here, but the numbers listed there would present one tenth of a percent of a duchy's land devoted to hard core agriculture does more than enough to feed the duchy's population. Presumably, the natural density of fruit-and-seed-bearing flora is such that the population gets along well enough on its own without the need for hardcore agriculture.
I'm not seeing where that "slavery" analog comes from. There's a large gap between "filled with spirit" and "sentient", and a larger gap still between "growing an orchard" and "practicing slavery". Besides, what of Kypiq use of trained animals? "Domestication" and "slavery" are a far cry from each other (barring the beliefs of certain animal rights groups). It is a relationship of trust - I protect and care for you, to ensure that you grow up healthy, and in return you aid me in my endeavors as you are best suited with your abilities.
On the subject of 10,000 Brudvir, the Brudvir have it even worse as far as foodstuffs go, to the point where their write-up specifically notes their tendency toward smaller settlements. Also, they're more likely to have a 3,000+ kcal diet.
It is worth considering that kypiq digestion, being particularly suited toward eating plantstuffs, is particularly suited toward efficient extraction of nutrients. It is important to note that the caloric content of fruits and vegetables is relatively low for humans because those fruits and veggies are packed with "dietary fiber" - indigestible materials that simply pass through the human digestion system. To a kypiq, much of that dietary fiber could be good healthy nutritive material.
Yeast would be a philosophical question for the Faedin to discuss once they have a handle on microbiology. At the tech level in question, leavening is simply something that happens to bread under the proper conditions.
The straw farm actually brings up an important point: grains are grasses. As anyone who's maintained a lawn can tell you, cutting the upper portions of a grass in no way kills the plant, nor does it hinder the growth.
Given all the above, I feel it safe to say that there's plenty the Kypiq can do to sustain a very large healthy population without any need to bend the tenets of their faith.