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?
In the Bluetooth settings for the headset, try selecting disconnect. This should disconnect it, until you connect again manually (I think). If that doesn't work, try switching off 'phone audio' in the Bluetooth settings for that device; but I'm not quite certain if you need to reenable that every time you want to use it.