Lecture slides before or after?
Students generally prefer to have the lecture slides beforehand, and you should let them make their own decisions about whether having the slides out during class will help their ability to concentrate.

On my first day teaching a class, I posted the lecture notes after the lecture, and I immediately got several requests along the lines of "can you post the lecture notes beforehand instead."

If you want engagement from the students, you should earn it by being a better lecturer.  Don't try to demand the students' attention by giving them fewer learning tools.