I could be wrong but i don’t think you can have default_port_association_mode to be OS specific, It can only be set per device or globally in the config.php with
For already added devices, i think you will need to edit each device -> SNMP -> change port association mode, go back to device settings and then re-discover the device.
You can set it per-device, but it is set by device attribs. The easiest way to set it is in the webui.
It will be changed on the next discovery (try clicking the rediscover button to trigger it)