The display manager for Xfce is normally lightdm. It’s obvious there was an issue with its installation and or configuration. Glad it’s up and running. If you can install inxi and post the command I gave you above it will helpful. There are two other backlight issues. I will let @manuel help you with the lightdm issue and maybe the others too but seeing the hardware info will help.
I still have to transfer the solution steps to my EndeavourOS.
I couldn’t get any further from the point: Boot up and go into tty with CTRL + ALT+ f2 and had to change the distribution to Manjaro before you answered. EndeavourOS stays black right after the first reboot and can’t be reached via tty. I prepared the stick with Rufus 3.11.1678 and the image: endeavouros-2020.09.20-x86_64 for tomorrow. The support here is really first class. It would be great if we could run EndeavourOS on the laptop. I simply like it more!
I think you’ll need to have it installed in order to fix any issue that you have. If you have trouble you can always boot on the live ISO and get on the forum and hopefully get advice. Then you would boot back into the installed system and make changes. Hopefully @manuel will be around to help with some of the more intricate things.
Edit: Once you get installed and is booting properly I think the backlighting issue might be resolved with the kernel parameter acpi_backlight=vendor added to the grub line and update grub.
Start the Live/System with endeavouros-2020.09.20-x86_64.iso and UEFI/GPT from Rufus 3.11
Update the Welcome APP from v3.4.35-1 to 3.4.36-1
Start the Installer and choose offline installation method
Location Europe/Berlin
Erase disk, with Swap (no hibernate)
Filesystem
6a. new gpt partition table on /dev/sda
6b. 300MB partition on /dev/sda with the filesystem fat32
6c. flag 300MB fat32 as boot
6d. create 230GB partition on /dev/sda with the filesystem ext4
6e. create 9GB partition on /dev/sda with the filesystem linuxswap
6f. Set up new fat32 partition with mount point /boot/efi
Install EndeavourOS on new ext4 system partititon
I have not restarted yet and remain in the live system until further instructions follow.