I don’t think the battery on a car sat nav would have enough staying power and would go flat too soon, that’s if you could find something to attach the mount to your bike securely.
I have a garmin 100, bike computer that I’ve had a long time, it has the ability to upload a route from something like Komoot, which I did for the first time yesterday, it was easy to follow despite no voice guidance and the battery still holds a good charge despite it being at least 4 years old, possibly a bit older.
something like this second hand wouldn’t be too expensive these days.