Rise 360: Can I make one lesson 'optional'?

You're correct: to require learners to complete the initial lessons in order with restricted navigation, the optional lesson needs to be at the end. 

To allow the learner to complete the course at the end of Lesson 5, insert a Storyline block that runs a completion trigger when its timeline starts. Use that to track completion when you export the course for the LMS:

The Storyline only needs 1 slide, which could have any message you want (e.g., "You have successfully completed this course"). You could even resize the SL file to be short and wide, so it doesn't take up the space of a full slide. 

Near that (before or after), you could put a button block that jumps to Lesson 6. Be sure the text makes it clear that's optional. 

Another option: You could lower the percentage of the course that has to be viewed so it would complete at the end of the 5th lesson. (If you do this, be sure to test it in the SCORM CLOUD. I don't know how Rise calculates the percentage.)