The proof of the pudding is in the eating. Honestly, to me your summer sounds completely awesome! But if the result is your kids are acting like turds, then, yes, it's time to switch it up.
I like the idea for more unstructured time -- that is something that is sorely missing in our current world. But I would also encourage you to load some more responsibility on your kids, especially during summer breaks. Since they don't have school responsibilities, they have the headspace to learn new things and be useful to you. IME, this actually makes them happier: humans were not designed to loll around and be constantly entertained 24/7 for months at a time; we want to be useful, to be productive, to contribute something useful to the other humans we care about. This is especially true for kids, who are trying very hard to learn all of the skills they will need as an adult, and so who are really internally driven to learn and contribute. So spend some of that extra time expecting them to be contributing members of the household, in ways that they may not have time/energy to do during the year. Thinks like cooking dinner, or planting/weeding the garden, or painting the house, or helping with home repairs -- obviously, the list needs to fit their ages and interests, but it should be something useful and beyond what you have previously expected them to do -- and then you should, of course, praise them ridiculously when they achieve whatever it is.
This is from personal experience, btw; at one point, I seriously thought DD was not going to survive until her 13th birthday, because OMG was she such an entitled twit -- here I was making her life so easy, doing all this stuff for her, and she couldn't even appreciate it, but instead demanded -- expected -- more and was resentful when I couldn't drop everything for whatever her minor little thing was? Yeah, sorry, that is not the kid I am raising. If you are that entitled, then clearly I have been doing TOO much, and it's time for me to back off with the "help" and "doing for," and time for you to pick up some serious slack. So we assigned her the task of making dinner once or twice a week. After the initial kvetching, she puffed right up -- with something real to be proud of, her attitude evaporated, and she went back to being the nice kid I knew. I was sort of amazed that it took so little!