If I ever felt like I was getting attacked too much I'd just focus defenses on one main settlement.
I don't see how that would help anything. You'd still get attacked just as much, you'd still have to go to a settlement every time you get attacked (which i imagine would be especially annoying in survival mode where you don't have fast travel), and that particular settlement wouldn't be any more likely to be attacked than the other ones.
Don't have too many settlements.
I'm sure the game would be
way more fun if people got attacked so often that they actively avoided expanding to new settlements solely because they couldn't afford defenses to make the attacks shorter.
Do you know how bored I get because settlement attacks are so unbelievably rare and when it happens the fight is boring.
If you think they're boring, why are you complaining that there aren't enough of them?