I guess I'm going to be somewhat a voice of dissent here.
To me, 7 and 8 is plenty old to start learning consideration for other people.
Not taking care of a beautiful gift is IMO inconsiderate to the giver. It's not about the thing, it is about the person. While it is probably true that they didn't mean any disrespect to their grandma, that just shows that they haven't yet learned that trashing a gift will very likely make the giver feel bad. So you have to teach them that.
Not picking up after oneself is inconsiderate to the OP. Obviously they should not be spending all day cleaning house, but just little things like taking their plate to the sink when they are done eating is totally within their mental abilities and is a start to teach consideration for others.
I understand the theory that children learn by example and if we model the behaviors we desire (for example, we put our plates in the sink) they will eventually catch on, but I also believe that all children are different and this method works better with some children than with others. I raised my dd very much with that philosophy but her personality is such that she doesn't really pick up on people's emotions and social cues and needs things spelled out to her more. She also much prefers to have things done for her than to do them herself. You know how people say that around 3-4 kids will want to try to tie their own shoelaces? She never did. She'd have me tie them for her today if I would (she is 8) LOL. It's just who she is.
So recently I have started being much more explicit in my expectations of her. Simple things, like making her own snack if I am busy, putting her dishes in the sink, setting and clearing the table for dinner, vacuuming etc. There was some resistance at first but she is fine with it now. I strongly feel that this is important for her, so that she learns consideration for others as well as pitching in to help the family. And as long as I approach it positively, not punitively, she seems to actually enjoy having a role to play.
OP, I don't have any advice because it sounds like you have tried a lot of things, and you have 5 kids whereas I only have one. I don't feel like I have anything useful to offer
. But I just wanted to say that I really understand how you feel, and that I think the behavior does need to be dealt with, not essential ignored and hope they will grow out of it. I kinda like the idea of sending all of their stuff into storage for a while, until you can regain some measure of sanity. Then there would have to be many heartfelt talks (if you haven't already) about how their actions affect other people in the family and how families need to be considerate of each other, along with clear expectations.