In countries like Australia which have tons of grassland they still use B12 supplements for other factors like the use of pesticides which destroy the B12 producing bacteria in the soil, antibiotics given to the animals which destroy the B12 producing bacteria in their gut, a lack of cobalt in the soil due to the type of rock from which the soil is derived, or due to the amount of rainfall causing cobalt to leach away from the top soil.

We overeat low quality meat. If we had higher quality meat then we would not need to eat as much of it to sustain ourselves. People also focus almost exclusively on muscle meats instead of organs, which means they have a hard time satisfying all of their nutritional requirements. It's a product of ignorance. Part of the reason we overeat in America (over half of people are obese) is because our bodies instinctively know we are not getting what we need, so we end up eating more to compensate. If we ate more nutritionally dense foods, we would not need to eat the same number of calories. There's a reason the leading causes of death in America are heart disease, cancer, chronic lower respiratory diseases, strokes, diabetes, nephritis, and flu/pneumonia. It's all due to poor nutrition.

For example, I buy my meat from local farms in MO and New Zealand and have it shipped to me. I stick to organ meats and I only eat them maybe two or three times a week. I can buy a few pounds of liver/kidney/etc from pasture raised animals and have it last a long time. You would think it to be expensive, and it is definitely higher priced than most meat I could find in supermarkets, but since I eat a lot less of it than I used to, it actually ends up being much cheaper. If everyone acted the way that I do, there would be more than enough grassland, and the animals would have much better lives.

Everyone knows you can get the vitamin D hormone from the ball of fire in the sky. In winter it's difficult as the cloud cover means you have to spend a lot longer outside where it's cold and unpleasant. I take this vitamin D supplement in the winter and it works quite well for me https://www.amazon.com/gp/product/B086NVQZB1/ . Tablets don't work at all for me, I don't think I can absorb them at all because I feel nothing after taking them.

Only if you live below the 39th parallel (meaning closer to the equator). People north of that (there is a nice map in the article) will not get adequate amounts of Vitamin D from the sun even if they are out in a clear day, except for in the summer months. And although the article I linked mentions fortified foods as a possibility, I wouldn't rely on it. The natural sources will be fish and unprocessed dairy products. If you want to supplement, then great, I only mean if you are trying to get adequate Vitamin D from food.

Proof? Been getting my vitamin A from pretty much carrots exclusively and never been deficient. I've also tried a preformed retinol supplement and didn't feel any different. I'm sure there are people who have trouble converting it though, but I wasn't aware this is generally true for most healthy normal people.

How do you know? I thought I wasn't deficient until I started eating liver and drinking raw milk, and my eyesight went from blurry at midrange to crystal clear. I thought I was going to need glasses and now my vision is excellent, and the main factor I can think of is that my Vitamin A consumption and absorption have significantly increased. (though my iron and zinc intake, among other minerals, also significantly increased at the same time, so it's hard to say for sure what I was missing) Also keep in mind that Vitamin A is fat soluble, so if you aren't consuming it with fats then it probably isn't being absorbed as well as you might think. (the Vitamin D supplement you mentioned adds coconut oil for that reason) We have to keep in mind that vitamins, minerals, and fats work together synergistically. That's why taking a single multivitamin doesn't work the wonders that people expect.

You can read about the limitations of beta-carotene here:

A summary of the major human studies that determined conversion factors for dietary beta-carotene to retinol is presented here, and these data show that the conversion efficiency of dietary beta-carotene to retinol is in the range of 3.6-28:1 by weight.

So in other words, it does vary for people, but even in the best case you are talking about having to eat three times as much in plant matter to get an equivalent amount of retinol that you would from meat, and it's as bad as 28 times. Don't take a retinol supplement. Try eating liver cooked in butter regularly. You are always going to get better results the closer you get to actual food. The only supplements I would take would be freeze dried non-defatted animal parts. (organs, fish eggs, etc) So something like this, this, or this. I take them along with essential fats every day. It's convenient when I don't feel like cooking meat.

Are you vegan or vegetarian?