The only bluetooth setting that can set in a regular configuration spreadsheet is the bluetooth_throttle. All others can only be set in the prefs.csv file, and nowhere else.
If you look at the original
Default Preferences where all the preferences are described, there is a color code as to where preference settings can and cannot be used.
| Must be set in prefs.csv or via QMP |
| Can be changed with voice command, or game configuration file's Preferences sheet |
| Can be changed on Mode sheet, Voice Command or Preferences sheet. |
| bluetooth_device_mode | ssp | | Bluetooth HID Device Selection |
| bluetooth_authentication_mode | 2 | | Bluetooth Authentication Selection |
| bluetooth_connection_mode | pair | | Bluetooth pairing control |
| bluetooth_throttle | 15 | ms | Bluetooth mouse report interval in ms. |
Those settings with the dark purple background must not be changed after power up. Once the Bluetooth device, authentication and connection mode are set, they cannot be changed without rebooting the CPU and the Bluetooth module and this only happens at power up.
| enable_usb_a_host | 1 | | Enable Host mode support for devices plugged into USB-A |
enable_usb_a_host is the only other preference setting like this. Once the USB hardware direction is initialized, it cannot be altered.
You need to remove any Preference settings that affect those preferences from any configuration files.
The QMP misc tab controls those settings and these are the settings that matter for using a serial connection over bluetooth:

Save those settings to the quadstick, wait for the quadstick to go "boing", then un-power the quadstick and power it back up.
Whenever the device mode is changed, any prior "pairing" is no longer valid and must be forgotten in the host and the quadstick re-paired as the new type of device. The quadstick must be paired within a few minutes after powering up. It will stop advertising itself after a while.
If you are having trouble seeing it as a new device on the PC, see if other hosts can see it. You do not need to pair to it to see it show up as available in other hosts, like a phone or tablet.
If it has been paired to another host, make sure that host is not on and in the area when trying to pair it with the PC. If the quadstick is actively connected to a host, it will not advertise itself for a new host to pair with. Probably best to have any other hosts that were ever paired to it forget their pairing to it. The quadstick remembers the last eight hosts.
Just to throw more complexity into it, not all Bluetooth adapters see all bluetooth devices. The built in bluetooth adapter on my desktop unit didn't work and I needed to replace it with a one that plugs in to a USB port.