We are experiencing similar random crashes of audio sessions with AVSpeechSynthesizer. After a lot of tests, it seems that the preUtteranceDelay is the problem if AVSpeechSynthesizer.stopSpeaking() is called during the delay period. I will fill a bug report and post the feedback ID here.

A TSI has been filled (15.10.2021). I will update this post when I obtain an answer or solution.

This problem has been solved in iOS 15. Now the first step has an instruction to start the route.