I realize that most of the time Scrum restricts the creative culture.
I'm so sick and tired of people saying "Scrum does this" or "Scrum does that" when in reality, they could read a short few pages of paper and see that they are wrong.
Your creativity is restricted because you do "design up front". Nowhere in Scrum does it say you have to have a designer. Or a design up front. Or a design at all. Scrum says your development team needs all the skills to turn a backlog item into a finished product. That's it. Any changes you make to this are on you. If you don't trust your team to do a good design by themselves and provide one for them, then it's you, not Scrum, restricting them.
Making team members feedback and discuss their product and design they're working on.
That is already a part of Scrum and called "Refinement" and "Sprint Planning". I wonder whether you do Scrum at all. What exactly are you doing in those meetings, if it's not feedback and discussion about the past and future product?
What I read from your question is that you have managed to restrict your team in a way that they start to show signs of "Don't care, not my product" and you don't actually want to officially change your restrictions, but instead want to find some way to uphold the restrictions, but have your team at the same time undermine and subvert them, by having hidden boosts of creativity, outside of the given process.
This is not going to fly with any process, Scrum or something else.
Since you use Scrum, the first step would be to read the Scrum guidelines and see what you should do. The whole restrictions around "Design" are of your own making and not a part of Scrum, so don't hide behind Scrum when it comes to evaluating them.
Then ask your team in a retrospective how to change things so they can be more creative. You cannot force creativity through a process, you can only remove restrictions and see what happens. And again: those restrictions were set by you, they are not part of Scrum.