You’re clearly wanting to stop the kids relationship with their dad to insert step dad and not have the hassle. I suggest you seek some support as you are still very bitter.
You call the kids Dad a Disney dad but what choice does he have when living 2hrs away? Of course he’s gonna buy them nice things and take them places. Plus if he does their homework/reading with them, that’s not really a Disney Dad. He’s taking the responsibility that he needs to.
Leaving clothes and toys at his is normal and sensible.
He only gets them 4 days a month, maybe he does bring them back slightly late but it’s at most 12 times a year. So what? You say it’s to get at you, maybe it is, maybe he just doesn’t want to say goodbye to his kids. Either way, don’t let it get to you. You can’t change other people’s actions just your reactions.
You seem to be deliberately planning stuff on those nights that make it all stressful. You can choose not to do that. Quick tea, quick bath, bed. You have the other 20 odd nights a month to see and enjoy your kids. Let go.
Why on earth would you plan a big thing for their step dad, on a night you know their dad has them? You set that one up to fail. What support did you give the kids to celebrate Father’s Day with their father? Did you help them make a card or help them buy a small gift for him or did all the effort go into the step dad? It’s the kids Dads day, I’d have expected the lateness.
I suspect you won’t listen to anything on this thread because you seem to be out to break their relationship. Let them be, unless he is actively harming them which doesn’t seem to be the case at all. If he is a dead beat dad they will come to that conclusion themselves eventually.