froggy4711 Android has acceleartes sensors, so it should be possible to generate a rule, if smartphone has a speed of 30km/h, to do something...
The accelerometer will sens only accelerations (with limits), so you won't have a "speed" value out of it.
You would need to sum constantly the accelerometer values over time to calculate your speed, or use 2 points from your gps (which is what your navigator does).
I am not aware of any application that monitors/try to calculate your speed, but I would start with applications like Tasker and see if there are some modules for your need.
Also, according to this, it seems that after Android 13+, there is no option to enable bluetooth without user action.
I remember there is an option to auto-enable wifi if near a known one (or something similar), I do not remember the same option for BT, but I might be wrong.
To be sure, I would suggest to check every option related to bluetooth, in case my memory is failing.