Thanks both for your replies!
@BBDDVV i looked into your suggestions, and what i found was that even after waiting 5 mins in the car, no audio output option would appear in the maps speaker options. likewise, going to bluetooth settings on the phone showed that it was connected and all features were enabled, but there was no option for 'HFP' which i assume means 'hands free prompts'?
i looked into the car's own bluetooth settings, and it showed that it had established a phone connection, but not a music connection. i selected music, and it force launched the music app on the phone and started playing through the car speakers. so i had to go and stop that, and *FINALLY* maps showed bluetooth output options.
i was hoping this was a one-off set of hoops to jump through to get it configured, but it turns out i have to do every step each time i start the car: wait 30s for bluetooth to connect > go to car settings and manually enable music playback > go to the music which force starts playing and stop it > go to maps and select my car as an output device.
bonus headache #1, sometimes the fails to detect the phone at all until i restart both the car and phone. thats rare though.
bonus headache #2, if i try to adjust the volume dial on the car for the turn-by-turn, it force starts the music AGAIN.
sigh
i'm almost *positive* my car used to just start auto-routing all audio through the car speakers within a 10-20s of turning the car on, and all this fiddliness started with iOS9. that said i'm sure the car shares the blame, as what works for one model of car probably breaks another.
but thank you for the advice, at least i know of a way to do it if i need it, i just probably won't bother as often because of the amount of time it requires.