Thank you for your replies. It is interesting that you got different results than me. On my undersanding, turning off the Bluetooth adapter definitely terminates the connection and thus the device can be used again for further scanning/connection.
I've been trying out a bunch of different things.
Sometimes, when the device is stuck on this state, I am able to retrieve the device from the regular getDevice(String macAddress) method. And then I can establish a connection, verify the BONDED state, read the standard Device Information Service, and even enable some BleNotify. However, I am not able to negotiate the MTU as the operation fails with a REMOTE_GATT_FAILURE due to “no resources”.
I'll activate autoNegotiateMtuOnReconnect and try again tomorrow, maybe you have some magic in there or I'm doing something wrong.
Other than that, I think I'll use the getDevices_bonded() function, filter for the device, and unbond it. Then connect again. That seems to work on the Toolbox V3 app and does not retrigger a bonding notification for some reason.