Is there a way to configure these files to be put on the 8TB drive instead? Or can I fix this by changing partition sizes? Something else? in System Information, it shows mmcblk1p2 is 28G, with 28G being used 100%.
The initial image that you wrote to the SD card has 512MB for /flash (boot) files and 32MB for /storage; the second partition should be removed and resized to 100% (32GB) on first boot. If you have run out of space, either something has dumped a ton of data to the SD card and filled 32GB up, or perhaps the remove/resize didn't happen and you've filled up 32MB not 32GB?
Boot params on the SD card in extlinux/extlinux.conf show boot=UUID=<string> disk=UUID=<string> .. if you check the UUID for the partition on the USB drive using blkid you can change disk=UUID=<string> to use that UUID and on boot it should use the USB drive for /storage and you'll start with a clean Kodi instance that you can move config over to. You can also mount using disk labels, e.g. boot=LABEL=LIBREELEC disk=LABEL=MYDRIVE assuming partitions are labelled or boot=/dev/mmcblk1p1 disk=/dev/sda1 style params. Note that you will need to use a Linux filesystem on the USB drive, e.g. EXT4, else the ssh daemon will fail to start since it depends on Linux filesystem permissions, and non-Linux filesystems (NTFS, exFAT) don't support Linux permissions.
In theory the SD card image can be written direct to the USB drive and it should boot from that too; although I have fuzzy recall that the Amlogic bootrom only checks for bootable USB drives in specific ports, e.g. the OTG port.