Of course it couldn't possibly be user error, could it? Maybe before coming out with guns blazing, it might be better to ask users on this user to user only forum for help by simply saying you're having Bluetooth issues and what is or isn't working and we users would be more than happy to try to guide you in the right direction. Just a thought........
I have iPhone 15 Pro Max. I have no less than 5 pairs of Bluetooth Headphones paired with it and not a single one is having any issues. Of course, I do know that in order to pair them with a new iPhone, they MUST be put in Pairing Mode per the manufacturers directions. And often if you have them paired with another device which is nearby and powered on, they may not necessarily pair easily, unless they are specifically designed for Multi-Point Pairing, which more modern Bluetooth headphones often can.
You want to try again and tell us exactly what issues you're having, which are pretty much guaranteed to not be an issue with your iPhone.