Can I copy my slides into other projects?

Yes, it is very simple. In the slide editor, in your work area, you will find a button that allows you to do this: the same one you use to duplicate the slides into other subsections.

When you press it, in the upper left corner of the pop-up window you can use the arrow to visualize the list of projects. There you may select the destination project.

Once you have selected the destination project, you will see its structure sections to choose which one you want to copy your slide to.