Yes. One thing that wasn't clear to me was where to actually get those files. I've been using the ones that are found in ~/tmp/deploy/images/h3ulcb/ after building yocto. Assuming that's the correct path, I'm going to try rebuilding yocto in case I messed something up in that process.
One possible indication of what the issue is: I looked through the Renesas Device Manual https://www.renesas.com/en-us/doc/products/rcar/R-Car_StarterKit_Gen3_H3_M3_DEV_Rev.053.pdf to find a solution and found that in the section on booting the addresses the board boots from are slightly different from those on the e-linux wiki (looking at page 238 in particular). I'm a machine learning guy by trade and my architecture knowledge is rather scant comparatively so maybe this is nothing/not what I thought it means.
Yes, /tmp/deploy/images/h3ulcb is correct path. Please refer to the No13 of http://elinux.org/R-Car/Boards/Yocto-Gen3#Building_the_BSP_for_Renesas_H3ULCB.2C_M3ULCB.
All updating methods are described below. http://elinux.org/R-Car/Boards/H3SK#Flashing_firmware Could you try again?
If "u-boot" still doesn't start up please attach the log when flashing the firmware.