This is correct. It doesn't matter how many planters you have, it matters the quality of the planter that does the harvesting. Be sure to turn off "all dwarves harvest" in
Incorrect. Harvesting gives experience, but the yield has nothing to do with who harvests and everything to do with who planted. Setting all dwarves harvest is inefficient, but can be a good idea if you're growing too much food for your growers to gather
Embark: I use the defaults because I'm lazy.
Farming: I usually have about 6-8 plots of 4x4 size, half underground and half surface. I can't always remember which crops are which, so aboveground I generally just plant the berries. I do a little local (d)esignate-(p)lant gathering in the first year to bootstrap the aboveground plots. Belowground, the first plot obviously gets plump helmets, then I usually do sweet pods and cave wheat in the next plots to get more booze variety. Obviously all the subterranean plots are set to plump helmets in the off seasons. This does require a fair amount of farm labor; I generally turn on farming for every brown-class migrant I get - soapmaker becomes farmer, lyemaker becomes farmer, milker becomes farmer, etc.
Brewing: I usually have just 1 carpenter shop, set to repeat bed/bin/barrel, which seems to be able to keep up with one still doing brew drink on repeat. I'll often keep a small food stockpile set to take only raw plants nearby, and a furniture stockpile set to keep only barrels. If the booze runs low it's usually because the brew job stopped because the barrels ran out because of a problem in the wood pipeline.
Fishing: The process-fish order does get cancelled (for lack of fish) more often than the brewing task does, but if you remember to check every season or so, it's fairly productive. My solution to the refuse problem is usually to have workshops and the main dining hall on one level, stairs down from each of those to a connecting tunnel on the next level down, and a big refuse storeroom (10x10 to start, growing as it fills up) a little ways off that tunnel, midway between the two. Short walk from the dinner table to the scrap heap, short walk from the craftdwarf's workshop to the scrap heap.
Hunting/Meat: Since immigrant hunters come with equipment, I just leave them to their task and hope for the best rather than trying to train & equip new ones. Their productivity and life expectancy is entirely dependent on what kind of local wildlife you have. I alternate training war dogs and hunting dogs, so any hunters that survive a season or two generally get a little buddy to improve their odds - (v)iew dwarf, (p)references, (e) work dogs, if you didn't know about that one. I butcher the fuck out of kittens, foals, calves, etc, and request every kind of meat from traders and buy it all, which means plenty of meat available -- not that you really need meat to run a fort.
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Either you're completely swimming in food or you're starving, amirite? I've fed fortresses of 60 dwarves with a 5x5 plot growing Plump Helmets and a single grower who also managed a second 5x5 plot of pig tails and dimple cups. I one time had a dedicated hunter who stocked the fort with over 600 meat in the space of a year on a map loaded with elk and horses, as well as thousands of bone bolts and enough totems to buy out a caravan (plus a legendary marksdwarf for running off the first few ambushes). Fishing is not that lucrative in my experience, it's very easy to overfish an area and the yield is pathetic compared to any other source of food.
My guess is you were either starving because all your farmers were inexperienced or you were so drowning in food you couldn't give it all away. One or two producers is more than enough to supply a fort until the later days, and even then if you use quarry bushes or boozecooking you can manage it with two growers and an army of processors.