Disable Bluetooth auto connect for specific devices

**The info** Android version: 8.1.0 Phone: BQ Aquaris X Pro. The phone ships with almost-stock Android. I am using Nova Launcher (not sure if important) **The situation** I have a Bluetooth headset on my motorbike's helmet wich supports both Bluetooth connection with the smartphone, and direct connection with another headset, but not simultaneously. So if I turn on my headset while my phone's Bluetooth is on, it connects automatically to my phone, before it connects to my partner's headset. Until now, I was just fine disabling Bluetooth on my phone when needing to pair with another headset. But I just got an smartwatch, and every time I disable Bluetooth on my phone, when I enable it again, the watch connects to my phone and starts a painfully long a-gps updating process. **The question** How do I tell my phone not to connect to the headset automatically?