I enjoy playing without invaders if I'm focused on industrial fortress design, or is playing around with designs and ideas. I use blocks everywhere and don't worries about gaps that's left behind them, because I look at it from dwarves' point of view, how should they care about what they can't get to?
If I want to fight something, I turn the invaders on. Forgotten beasts comes comes on invasion timers, it seems, so if you walls yourself off aboveground, you could find those terrors as dungeon bosses of some sort
You can turn off caravans by removing ( I believe ) active seasons tags, so that helps to reduce managing them, if you're worried about it. I -believe- invasions depends on those tags to time their invasions too, so if you removes them, then you'll only have to face forgotten beasts and titans.
I've honestly not had much problem with military, but I also tend to go with combo of trap and fortified markdwarves to grind up anything that comes in. Death mattering a lot is pretty much the one social element that makes happiness important. Death by old age happens too ( and tend to outnumber combat and accident deaths in long run for me ).
I don't think there're really a 'core' gameplay element, it's supposed to be you running a city or fortress within the world. And if it so happen that someone's out to get you and you're in the right place...
So there're the fortress where you protect your dwarves the best you can ( or badly if you have some strange goal in mind ). The building game where you design and build up what you want. The industrial game of designing workflow with workshop and stockpile arrangment. The economical game of trading enough to send traders home twice as rich with as little material used as possible (This one don't really works well because everything is fixed-price
).
So regardless of how it goes, if you enjoy a certain way, then it's your pleasure!
You might enjoy some mods too, if you're looking for more internal details, instead of facing the enemies.