So I ran into the same problems described here: Install Ubuntu Bionic 18.04 on MacBook Pro 2018? Just that I've got the 15''.
Booting works (after I disabled Secure Boot and allowing to boot from any external device with any bootable system in the System Recovery)
I'm currently stuck at the repartitioning of my drive because ubi-partman keeps crashing with a 141 error code. I installed gparted on the Kubuntu live system to check the drives and it seems that only the USB-drive from which Kubuntu is running is detected. The 100 GB Mac Partition nor the 900 GB FAT formatted partition I want to use for the installation are listed anywhere. I tried installing e2fsprogs on MacOs and formatting the Linux Partition with ext4, but I get the same result.
What else can I try?