I went back and looked at your previous thread about having issues with turn-by-turn directions playing. It is peculiar that it doesn't play over BT when connected to your auto. I use GMaps over BT and haven't had many issues. Two things on that note; While I see that you are satisfied with receiving directions via the phone vs the car's stereo but the two things I would check are, GMap's volume and Spotify's pause for notifications setting.
For GMaps, open the app>touch your icon (upper right corner)>Settings>Navigation settings>Guidance volume. If possible turn it up at least one setting or just go the max, at least for testing purposes.
For Spotify, check the app settings for an option that pauses for notifications. I don't Spotify so I don't know the ins and outs of the app but I would imagine being the polished app that it is, it will have an option to pause for notifications or other app sounds or interruptions or something along those lines.
Then check if it works, if for nothing else S+G.
As for how to automate defaulting your GMaps to your device, use a Bixby routine. If you open the settings menu on your phone and type Bixby in the search, Bixby routines should be the first suggestion or at least among the top couple. If you haven't enabled it or agreed to the TOS or whatever it is that it asks for the first time, agree and start building your routine. "If" would be Bluetooth. Select your auto>Connected>Done. "Then" would be Sounds and vibrations. Select Separate app sound (this option has a BT speaker for an icon)>Phone (is the default thing to pop up but right next to it is Bluetooth so select the one you want)>Select the apps that you wish to use the phone when phone is highlighted and select the apps that you wish to use when BT is highlighted. Bixby should kick in whenever your connected to your auto and the app(s) you selected audio sources for should default to the option you chose. Other audio not selected will follow the device. What I mean is if the device normally would broadcast via BT sounds will come from the BT device or vice versa.