I purchased Duet APP to use with the iPad 12.9 with a rMBP 2015 when I travel - it uses the TB / charger cable to connect.
Suggest you try the demo version first - I have not been able to get it to work after upgrading to Mojave - it gets stuck in mirror mode - which is not what I wanted - I was looking to extend the desk top.
FWIW - Does not work well for me - gave up on it after upgrading to Mojave
I had stopped using duet on my 2017 iPad because it will not charge while connected. Just tried it again with my 2018, it will now charge while connected. However, the touchscreen doesn't work. The touchscreen used to work, you could essentially run Mac apps with a touchscreen interface. It appears that no longer works, duet lets the iPad work as a second display, but only with trackpad support. I'm not going to use duet because it doesn't support touchscreen anymore and the connection is flaky and disconnects after like 5 seconds if you switch to another app on the iPad. Duet "works" but because of these problems it is not useful to me.
I used my first and second gen 12.9” iPad Pros with both my 2014 MBP and work Dell laptops on USB to lightning cables with Duet. But, over the past year or so it became less and less reliable.
I just upgraded my iPad to the 2018 version and my MBP for a 2018 Mac Mini. I’m now using Luna Display with either WiFi or USB-C connection. I used my Samsung tv to help set up the system. But, since my usage is 95% iPad and 5% Mac, I am using the iPad as my only display.
It is a great solution, works surprisingly well and gives a glimpse of what an iPad running Mac OS could be.
I highly recommend it as a second display and if your use case is similar to mine, it works well as a single monitor too.