Unless you want to build up your armies strength I wouldn't worry about holding back on taking the capital. Only by taking the capital does the harem stuff kick in.
Really you should be fine with just that doing those two things. But the game does have some hidden stuff to trigger some NTR events. I don't mean to spoil anything but to provide a little more context I'll use the girl that takes care of the MC as an example and I'll put it spoiler tags.
You don't have to drag anything out unless you want all the girls in the harem. For example the thief girl you must follow her story chain to allow her to get put in the harem.
Some other things to take note of would be building up their loyalty will slow down the process of them being seduced by the king, but that provides more of a chance to see a slow burn to their corruption and their change in response to the king and MC. If you want to just cut to the chase, having them lose to the enemy army and get captured will do the trick. But be careful with this as the king will no longer want anyone that's been fucked by another dude so they'll never be called to the harem. And the units will work for the enemy if they aren't saved. They can ramp up their power and do some real damage (The MC has to be alive/not captured so the game continues).
Once the Harem feature is introduced the king will have an expectation of a certain number of women in it, you'll be able to decrease the number to it's minimum and he'll more often send out his guards. If you don't want to do that then just wait a while. He increases the amount of women he wants especially if the amount of heirs he sired doesn't meet the demanded threshold. The game will explain this a little better.