I use the Ride with GPS app on my iPhone. If you pay for a subscription, RwGPS will give you spoken turn-by-turn navigation. I use it with a bone-conduction headset so I can hear the cues and road noise both. I do keep my phone mounted to my handlebars, but it doesn't need to be there. I've ridden thousands of miles with this setup, and gone for rides that are 12+ hours at a stretch. I recommend it.
Turn-by-turn navigation with RwGPS is different than it is with Google Maps: it's essentially the electronic equivalent of following a pre-made cue sheet. If you go off course, it'll play the sad "off course" noise (and it detects you going off course very quickly), but it won't reroute you on the fly.