Controlling when bluetooth connects

I have two bluetooth devices in my car, an Amazon Echo Auto and an external speaker/microphone combination (an Avantree CK11). The bluetooth native to my car is primitive so it is dangerous to search my contacts while driving and hands free is not natively available. So I unpaired my iphone 11 pro and installed the bluetooth Echo Auto and the bluetooth external speaker/microphone. My intention was to use the Echo Auto for entertainment through the audio line input in my car stereo and the speaker/microphone for Siri, navigation and telephone calls. By themselves all of the devices work as desired however when both devices are powered the Alexa app in my phone will connect directly to the speaker/microphone and route audio to it rather than stay connected to the Echo Auto. The requirement to enable Alexa access to all of my personal information makes me reticent to use Amazon exclusively for communication, and texting is impossible unless Amazon has pre-arranged authorization from the recipient. So I'm trying to prevent the Alexa app from connecting to the speaker/microphone and instead stay connected to the Echo Auto. Any ideas? Many thanks in advance.