If the OP is happy using linux/Ardour, why not just buy a new SSD for the laptop(quick google shows £10-£15 for a new 120GB SSD drive) which will likely have better sustained write speed than what's in there already, and make a fresh install of whatever distro he is comfortable using. According to Sound Devices calculation tool, 15 tracks @24bit 48Khz requires less than 8Gb per hour
https://www.sounddevices.com/audio-recording-calculator/
Recording audio really isn't particularly CPU intensive if you are simply streaming it to disk - simply set buffer sizes to maximum and turn off monitoring in the DAW.
When everything is recorded, no faffing necessary, just copy it across to the big PC - just make one custom template in Ardour with all your tracks named, and audio routing assigned and then use it for every recording.
Before going this route I would hook up the XR18 to the main PC and make sure you can see all the outputs (Behringer don't publish a mac driver so this should also work fine on any linux distro that plays nice with usb class compliant audio interfaces).