My Garmin and player works just fine on my other vehicles as well as my gf cars. My phone will not even show up unless the BRP software is installed.
IMO, Escorial, you have a fundamental misunderstanding of Bluetooth. BT is a communications system, i.e., a method to move data from one device to another. It is nothing more, nothing less. Think of it as simply a replacement for a physical wire connecting two devices. But that is only 1/2 of the equation. The other half is the program at the sending end that collects and organizes the data and the program at the receiving end that takes the data and deciphers it into a usable format. In the case of BRP Connect, or Go as it is called now, the Go app on the phone takes the various sources of data such as phone operations and audio broadcasting and converts them into a format the Go app on the Spyder can interpret, and then sends them to the Spyder. That's why you have to have the Go app on your phone. It is not strictly a case of Bluetooth incompatibility, or failure to implement the latest BT protocols. It is a case of data handling incompatibility. I'm sure that's what you are seeing with your player. Although you may be seeing an incompatibility in the specific BT transmission protocols used by your player and BRP Go. If I recall correctly from the website for your player they use a version of BT that is intended to transmit data in a very high fidelity format. As I said earlier, probably no one other than you is expecting to hear super high fidelity audio played over the Spyder system. Since it won't be used, why build it in, especially if there are licensing fees involved. In addition, BRP wants to restrict for safety how much the rider can fiddle around with his/her phone while riding, much more so than a car manufacturer does. To do that they implemented a software, (data handling) system that requires significant control over the data.
My Samsung phone connects quite nicely to my Lenovo laptop but it will not transfer photo files from the phone to the laptop. Apparently there have been changes in the format of the data the newer phone sends that the laptop cannot interpret. But as I say, there is a very good BT pairing between them. My Samsung tablet will not connect to my 2016 Nissan Maxima. I think there is a lot more incompatibility among BT devices than what you have experienced.
Don't forget, there really are proprietary versions of some BT protocols. For example, it's my understanding that Sena group communication protocol will not connect to Scala group communication protocol. But they are both BT. Sure, it would be nice if they both used the same protocol for compatibility, but that isn't the capitalistic way.
If you think about what you're dealing with in terms of two separate aspects, transmission and data processing, I think you'll gain a better appreciation for the cause of your frustration.