This is almost exactly how I play DF, with these exact modifications, so I think I can answer most of your questions.
Crops:
-Plants take 1-2 seasons to grow
-Plants only grow in single seasons to force crop rotation
-All plants require some form of milling or processing and then cooking before they can be eaten
-Drink base values dropped (less euphoria from excellent booze, yes?)
-Plant byproduct values leveled
These are all possible and easily done.
-Little less yield per seed,
The tag that controls this is broken, so this is unfortunately not possible.
little more plant required per brew
That's hard coded, unfortunately, so also not possible.
I increased the [GROWDUR:X] to 1 season, and had each main crop down to only 1 growing season. Even with 50 seeds of each, they're red on the crop selection list as if I have no seeds. What?
Think about it this way - if the plant can only grow during Spring, and spring is 1008 units long, and the plant takes 1008 time units to mature, then it doesn't have time to mature - because even during time unit 1 of Spring, there are only 1007 time units left and the plant needs 1008. In other words, you need to make growdur slightly shorter than a season in order to have time to actually plant the crops at the start of the season.
Enemies:-Removed [BABY_SNATCHER] tag from goblins (every baby snatcher incident now a full blown invasion)-Process trigger tags lowered from 2 to 1 (goblins invade within 1-2 sleep cycles from the start guaranteed)
That works well. You can also replace [INTELLIGENT] with [CAN_CIV][CAN_LEARN], this removed the ability of the goblins to speak, so they can not make treaties, so they are always at war with everyone.
I was considering giving goblins better equipment, but then you get better equipment from them. How can I make the goblins sturdier?
I find the best way is to make them physically sturdier - up their [SIZE:X] and give them some natural damage resistance [DAMBLOCK:X]
How can I mod it so it's guaranteed the Dwarves start at war with everyone?
The only way to 100% guarantee it is to remove the ability to speak from all other races (an idea just occurred to me - try removing speaking just from the dwarves instead of everyone else?) but this makes them all hostile to _everyone_ including each other, so some race is almost guaranteed to get killed off during world gen. A better solution, but one that doesn't work every time, is to give dwaves opposite ethical values than everyone else, and make everyone else match. IE, split the ethics tags in half, give dwarves UNTHINKABLE in half and ALLOWED(?) in the other half, and make the other races all mirror images of the dwarves.
Can a goblin, elf, and human force siege you all at once?
I have no idea! Find out!
Could I make the humans and elves like the goblins, with the process trigger tags so that they will be guaranteed to show within one season?
If they start at war with you, changing the process trigger tags should work fine.
Good luck!