How to track course completion if there is no assessment in a course.

The most likely reason for your issue with completion based on slide views is that there are probably one or more slides in the set that the user does not need to visit in order to reach the end of the module.  So setting completion at 100% of slides viewed in that case would never allow completion to occur,  You are usually better off working out exactly how many slides MUST as a minimum be visited and set the slide views at that specific number (or maybe even one less to make sure) rather than using a percentage.

If you decide to go with the Quiz Passed option, but don't want to have any quiz questions, just add a button near the end of the project and set it to Report to Quiz with a score of 1 or more points.  Set it up so that the learner MUST click this button for some reason, even if only to get to the slide that tells them they have completed the module and what to do next.  That will then give you completion.