TL;DR: Make sure to use ext4 formatting for the Kicksecure partition.
FWIW, after hours of struggling, I noticed Patrick’s post: “So I create a live ISO with openSUSE’s …” So I tried a minimal Ubuntu install on the USB drive, then replaced it with Kicksecure and it worked!
Upon further research, I realized that I had misinterpreted the instructions regarding the EFI partition. It turns out that I needed to ensure it was FAT32/ExFAT for proper functioning, but I incorrectly assumed the Kicksecure partition required this format, too. Actually, the Kicksecure partition requires a Linux disk format, and I chose ext4. I was able to verify this was my issue by successfully installing Kicksecure on a second USB drive using an ext4 file system for the Kicksecure partition.
I hope this is helpful.