In some sense it seems you overthink a little, considering what I saw looking for "meat diet" on google, maybe not surprising.
Indigenous people, which live in permafrost areas in the north, are well known to sustain themselves predominantly on a meat/fish diet. I would say 100% of food is that, but if they are bothering to eat some moss or other plant matter which can be found in summer, instead of drinking delicious blood or crispy/poppy(making pop sound) eyes then idk, maybe your people can do the same if they need it.
There is plenty of edible wild grass with low nutritional value but which may provide fiber for those who need it. That salad that grows so wildly in aquaponics is of the same type - fast grow cycles easy to grow.
- I have to point out an interesting moment/fact, it helps those people to have such diet from an early age(maybe very early one(i guess), in form of sucking meat(is a thing)), there are some changes in the development process which may disappear in adults which didn't support that type of diet from an early age. Thanks to that they can eat, what other people(at least a good portion of them, can't say all) can die from. There was whole research on that matter because of one case.
This is https://www.livekindly.co/meat-loving-argentines-considering-vegan/ what I mention in comments, ignore vegan stuff, just didn't bother to look anything better
A traditional Argentinian diet is meat-heavy; the national dish is asados, which is literally a variety of barbecued meat. Foods like morcilla (aka blood sausage), steaks, ribs, chinchulínes (the small intestine of a cow), chorizo, and mollejas (organ meat), are cooked together on a large grill or open fire.
Nomadic tribes, also not known to be so much focused on plant matter, to eat for themselves. I heard they were doing well, but know less about them.
So as I said the situation is far from exotic.
But an interesting low-tech thing I remembered is fermented meat. Salami is fermented meat, so as there are other well-known, I would say mundane examples, fish included. https://en.wikipedia.org/wiki/Fermented_meat
The process of fermentation may be used to render edible meat that would otherwise be poisonous to humans, as in the case of the Icelandic dish hákarl, the fermented meat of the Greenland shark.
There are other, lesser-known examples of more exotic cases of fermented meat, this one from wiki so as other cases (at least 3 more I know of, oh another one I remembered, but won't admit I know it(but it makes sense, but it bad, lol), and I'm sure there is much more)
What is interesting here is making inedible meat to be edible. And here we have the beauty of microbiology as a solution for everything.
Besides preservation goals, and remove poison, kill parasites(?), there is another goal to convert some portion of energy, through fermenting, to raise increase what it is lacking through bacterias which produce that something for themselves, and you just eat them or their remains after. Meaning if there was nutritional disbalance it can be one of the universal ways to fix it.
Developing proper cultures, to seed fermentation in the direction you like (producing strawberry smell or banana flavor, lol, or reducing increasing vitamin C presence, or producing fiber, etc) may become a goal. And I have such a gut feeling - not such a distant one, and maybe if to dig the topic it may be already what happens with those fermented products in this or another way/degree.
If you need to beautify your work with details, with smart dietary aspects and words or medical terms, unfortunately, can't help with that, and sure it would be great if someone would provide a condensed list of those terms and smart words to be used and later assigned to different body parts of your game, yeah, so many opinions and diluted materials if one is looking on google - yeah definitely a problem, but not competent to help with that.
If you looking for solutions, then maybe do not strike out low tech plant growth, it is not impossible and there are all kinds of those to select from, not only those you see on shelves of your markets. If someone needs it, they can harvest/grow some, even if they are mostly in hideouts in some region, they can harvest wilder varieties of stuff they need and help them grow inconspicuously. Berries bushes as an example, useful seed propagation over some area, trees, bad plant elimination, etc. You do not have to tilt the soil to get your plants. And it is just a supplementary dietary component, not a staple food, there is no need for fields of it.
In general that creatures meat is just another form of energy, which can be used in more complex setups, which vary from low tech to more advanced setups, to convert it to regular food of any variety you prefer(not exactly, prefer more like which is more suitable than source one). That BIOS-3 like an experiment from China a few years back - they had a special assortment of plants, worms - and they called it a success. So complexity maybe not necessarily of technological nature but in a form of small bio ecosystems, which are adjusted to be nourished by the materials you get, not requiring sun and all that.
So there is more than one way to skin a cat if required, but what is not possible is to tell which parts of those creatures == which dietary necessities. If we assume it is a typical assortment of creatures, then anything goes, with a small percentage of exceptions including do not eat yellow snow and braun one, leave it to plants and deers.
wasn't exactly joking about banana flavors, classical soy sauce, which is fermented for 2-3 years, I'm told it contains a rich variety of flavors/tastes which are not typical for source matter, and unique to the place where it is made.
selective breeding of cultures does not require complex equipment, or I mean to accentuate one aspect - depriving mixture of strands of oxygen and having a surplus of potentially digestible matter forces bacterias to become more "creative", increasing complexity becomes an advantageous strategy for them, this way they can extract more and last longer. And to implement more complex cycles they need more complex tools, which means they produce a wider variety of biological stuff and byproducts. And if it is edible it increases chances for humans to get what they may need or what they would be lacking be that food consumed directly.
there is another moment - a colony of bacterias and stuff like, a mix of different strands creates a system, which stabilizes properties of that mix, it one of the mechanisms of how our guts microflora works. Meaning if one of the testing cans got you edible results, then it may be expected to produce similar results in the future. It makes sense to work with the stuff in the way selective breeding is done with animals(I mean perceive that mixture as a small animal). If one is at a low tech level, such an approach can be an option and it few steps behind more advanced approaches, so one can get next level in that relatively simple. And then the next big big jump is gene editing, but it is far from simple stuff.
