If all I ever grow are plump helmets, is there any point to cooking them?
Since cooking destroys the seeds, you would have to be very careful (never cook so many that you lose all your seeds). I'd just say "no". I never allow direct cooking of plants, unless they are above-ground plants that I will never be able to grow in my biome.
Should I go for Easy meals or Lavish meals, if I have a legendary cook but only one ingredient?
If your cook is already legendary, might as well crank out the lavish meals, since you no longer care about the experience gain for making twice as many easy meals.
For instance, if my dwarves only drink dwarven wine (from plump helmets), what kind of beverage should I make to get rid of the "tired of drinking the same old booze" thought?
If all your farms are underground, your booze crops are plump helmet, pig tail, sweet pod and cave wheat. I personally never allow brewing of pig tails, because they're too important for clothing (and bags, and hospital use, and ropes). So, I disable pig tail brewing, and allow the other three. That gives me large stockpiles of dwarven wine, rum and beer.
If you have above-ground crops, then the number of available booze plants is potentially enormous, but it will depend on your biome(s).
Also, is it feasible to clothe all your dwarves by yourself? I'm on year 2 and my 64 dwarves all have slightly worn x(clothing)x, so I made a rope reed farm, though I don't know how many rope reeds I'll need.
Each piece of clothing wears out after about 2 years. Each dwarf needs at least a torso covering (cloak), lower body covering (trousers) and foot covering (shoes). That's 3 pig tails (or rope reeds, etc.) per dwarf every 2 years. You can decrease that by putting civilians into military squads and issuing armor -- armor never wears out.