You either have to go all in, or don't blanket at all. A too light blanket will only serve to squash the hair underneath and lower the insulating power of the horse's coat.
With an unblanketed horse, you must provide three things. 24/7 access to fresh drinking water, a waterproof/windproof shelter big enough for ALL the horses and 24/7 forage. Grains are un-necessary as horses heat themselves through fermentation of the forage in their gut. If a horse needs a weight booster in the winter and you have no option to bring him inside at night and blanket him during the day, feed beet pulp or rice bran.
With a blanketed horse, you must be sure the horse has adequate blanketing to make up for not having a coat. On my horse who is shaved, this means when it gets below 0F he needs two heavy fill blankets (about 500-550g fill total) and a hood. With a blanketed horse out during the day shelter is not needed, but it is nice and they need to be fed 1-2 times during the day, access to water is also nice, but if they are only out for a few hours they can drink when they are inside. With a horse out 24/7 they still need a shelter, water and hay.
They say money doesn't buy happiness -- well happiness doesn't buy horses!