Navigation by voice works quite well for me. Just connect the phone to the helmet's intercom and turn on voice navigation on e.g. Google Maps. If you have that option, I'd give it a go. Granted it might not be at it's best in "spaghetti" cities, but it has served me reasonably well in those scenarios as well.

The problem I have with bike navigation is that it's often something like "continue north". I ... have no idea where north is in a random location :)

This is reminiscent of the Fitbod app on App Store/Google Play. If you're looking for inspiration, check it out.