You're welcome.
About adding more to their diet, it sounds like you're doing good by them already; still, anything healthy for them like fruits will benefit them.
Raw apple for example can help with diarrhea, I'd just get cheap boxes from Co-ops etc and smash the apples on the ground for them to peck at. The pectin in apple seeds also detoxes heavy metals from them, so if they've been exposed to any they will also benefit from that. ACV would most likely cover that for you though.
You may wonder where a chicken could get heavy metal exposure but it's actually very common, there are many sources available from lead paint in old houses to soils in yards where someone once parked a car with a battery leak, or any soils around sheds or workshops, or any/all areas besides main roads or frequently used roads. The lead levels found in normal suburban areas are 'surprisingly high' according to researchers, just from constant exposure to exhaust fumes from vehicles over the decades.
I'm not sure about it being too useful to add garlic to their water, it seems to ferment it really quickly, which is an acquired taste they will learn over time but avoid initially, and for general health they should have a source of clean water preferably at all times though it's not like adding ACV or garlic to it's going to harm them. At least they'll get a taste of it.
If you're not yet soaking or fermenting their grains, it's a very good thing to do for their health as well as your feed budget, even just soaking it overnight makes a huge difference in how much of the nutrients they digest from each grain or seed, and you see the benefits in their health quickly. Soon enough they won't want unsoaked grains, lol.
I got them used to garlic by throwing it into their feed, so they could pick around it and generally get the taste of it that way before they decided to pick it out deliberately. My chooks went off Chinese garlic after the Fukishima incident, which is odd because supposedly it's still testing as safe... But either way I give them different countries' garlic now because they won't eat Chinese any longer. If they won't eat one strain of garlic it may not be the best... We've bred some commercial cultivars into almost alien representatives of their ancestral plant types, complete with widely varying nutrient composition differences from the originals, so it pays to shop around and not assume any plant or food has the nutrients it's credited with in literature.
Most animals species have a 'feedback' mechanism that tells them if something they're eating is good or bad for them via their physiological response to something they are tasting or consuming. With some chooks it's so fast they can literally peck and instantly spit out something, not swallow or hold, and they will know whether it's good to eat right now or not; with some other chooks, it takes days, even weeks before they're sure. I'm not sure if the delayed response is due to low levels of harmful chemicals currently in the plant/foodstuff tasted, or a weaker ability in that specific animal. Certainly some chooks do not seem to have any sense about what they eat. I believe it's adaptable and they build on experience and get quicker responses the longer they practice/stimulate and reinforce the mechanism, like all instincts, so if you carefully expose them bit by bit to a more natural environment they adapt and learn to be careful. Some people put chooks straight out onto long grass when they've never seen grass and this is the scenario wherein you can almost bet safe money on at least one of them coming down with a blocked gizzard or crop impaction because of their inexperience.
You will probably see your chickens do the peck-and-spit as they encounter new foods and plants. This instant feedback-loop/diagnosing ability is especially important for animals like chickens and goats who often eat plants which may be toxic during certain times only and safe at other times. I reckon chickens are the goats of the poultry world anyway, can't offhand think of another bird that is so adaptable, lol. They taste a bit of everything. Just as with baby goats and some breeds of primitive sheep, they learn by tasting everything and what doesn't kill them makes them wiser. They have large and usually powerful livers for just this purpose.
I use kelp powder or granules for the bulk of their vitamins and mineral supplements, it's incredibly nutrient rich. You can buy it from most feedstores or produce stores, and just a pinch per bird per day will make a big difference in their health, and also in calmness and instinct levels, and the more generations you do this for, the healthier each one will hatch out. They just get faster and healthier with each kelp-fed generation.
It raises I.Q. in any animal or human that eats it and prevents that terrible moulting so many chooks do, because it regulates the endocrine system so the feathers shed and are replaced in a staggered pattern, not all at once.
I give kelp to dogs, cats, horses, cattle, sheep, goats, chickens, turkeys, geese, even the budgie gets some too, lol. And myself and the kids. Specifically children with Down's Syndrome benefit greatly from kelp, it contributes to them developing physically into a normal appearance and of course mentally it helps anything that eats it. I recently saw a strawberry patch cropping four times in one season just because kelp was added. It's really great stuff.
It will also give them stronger and shinier feathers, and make them more fertile with a lower incidence of deformed sperm or eggs (which are simply byproducts of better health really, no matter how you achieve this health --- you could use some of the other very nutritious land plants or supplements, like stinging nettle plant. It's simple to grow, dry, and add to their food, and very nutritious).
Also, if they were not showing their true colors due to lack of nutrients, they will within a year show their true colors. Lack of iodine and copper and other trace elements changes the skin, tissue, feather or fur coloring, irises, everything is subject to nutrition levels. I've seen 2-year-old pure white birds change into black/red/yellowish mottled and streaked birds when put onto a diet containing kelp. Everything changed. Their eyes, beaks, legs, claws, feathers, skin, crests and wattles, eggshells, all changed after a year on kelp.
It's not uncommon for livestock to show weaker/faded coloring or even pure white coloring due to lack of complete nutrience. Humans put onto kelp inclusive diets have regained their natural hair color after going grey or white.
There are a few species of kelp but whatever you can get at the produce store will most likely be sufficient; it's sold there for breeders and hobbyists who want the very best health in their pets or livestock, but plenty of people don't yet know how good it is and are unwilling to experiment, which I don't blame them for, lol...
Anything that regulates and supports the endocrine system like kelp does will help in everything from social manners to reproduction to longevity and immune system function. When total nutrition needs are met, animals don't care about overstocking, even normally antisocial animals. It helps, of course, if any other issues are managed as well like removing bullies and liming the ground to help clean it and burn parasite eggs and pathogens.
I believe kelp's one of the main reasons I've had so little disease in my flock. Here I can get a kilo for $6 and it lasts a flock of around 100 chooks for about 6 months, but I could buy it in bulk for less per kilo. Unlike many other supplements nothing in it is synthetic or denatured, so it functions in the body naturally. Sounds too good to be true but if you want to go the natural route with your animals I highly recommend you try it. I've experimented both with and without it and there's no comparison, I don't intend to go without it in future.
Best wishes.