This https://1drv.ms/u/s!AvHY_kl4hMB4gQHfDAh2SYG_Q4Bt Xubuntu 18.04 installs and works pretty fine on Raspberry Pi 3 B+
Follow the instructions here: https://ubuntu-mate.community/t/aarch64-on-raspberry-pi-2-rev-1-2-3b-3b/16853
note: The installer is designed to be run from a flash usb drive and installed to another device (SD card, hard drive).
To make it lightweight, you will have to manually remove all the packages that you don’t need.