I always left my own DD to pack their own stuff at that age.
The rules were, a couple of days before, ask them if they have anything they want to take away with them that needs washing it needs to be in the basket by 10am. Anything not in there will NOT get washed and they won't be able to take it. So its on them to make sure it is.
Then they were told what time we would be leaving, for example 9am, on the dot. Anything not packed and ready to go into the car by then would not be coming with us.
Then we left them to be responsible for themselves. If they didn't put it in the basket, it didn't get washed, if they forgot to pack something then they forgot to pack it, its on them. I will admit to giving basic reminders now and then though like 'don't forget your sunglasses' and just checking they had their phone chargers for example.
Sometimes, my youngest DD especially, would be up at 6am on the morning we were leaving and do ALL her packing at that time. To be honest, if that is how she wanted to do it, then so be it.
She was always ready to go when we said, and never forgot anything.
It is much better to just give them the responsibility. Tell them how it is going to be and leave them to it. If they don't pack until literally an hour before you leave then so be it. As long as they are ready. Takes the constant stress and battles out of the equation.