It can get better, but only if your stbDH is prepared to change. Does he know how you feel? Have you told him?
You say that you love your DP, but do you respect him? If he refuses to parent his DC's, allows them to show him and you disrespect and chooses to indulge his DC's rather than contribute towards the household essentials, how much longer do you think you can love him regardless?
His parenting (or lack of) is an integral part of who he is - and even if 50% of the time, your relationship is great, that doesn't mean you should ignore the other 50%. You may find that your DP's disney attitude leads to one or more of his DC's choosing to live with him (and you) full time when they are older - so you would lose the 50% of your DP that you love as he would be DisneyDad on a fulltime basis!
You are right, it is not their fault. They are displaying the behaviour they do because it is rewarding rather than leading to negative consequences for them.
Smashing a window relieves frustration, so is rewarding for the DC. Whereas in my household, the negative consequence of smashing a window would outweigh the reward several times over, so my DC/DSC are unlikely to do it again - if there are no negative consequences for that behaviour, then the
DC is going to repeat it the next time they need to relieve their frustration.
The question is, what are you prepared to do about it? Unless you say something, nothing will change. If you do say something, your DP might make all the right noises, but won't actually follow through. Alternatively, he might tell you that is the way things are going to be, and you have to accept it. It is very unlikely that an established pattern of behaviour will change overnight, and even if your DP is committed to changing, you can expect to have many more awkward weekends and difficult conversations, reminding your DP of your boundaries. I would never, ever advise anyone to marry a disneydad. I would suggest that you put plans for marriage on hold until he can demonstrate his commitment to parenting his DC's and by respecting you by ensuring that his DC's respect you through their actions and words.