I recently attempted booting off of eMMC with sysimage-v3 as well. Turns out that there are some missing tweaks necessary for reform-boot-config before this can work without any manual adjustments. In the process I also extended the reform-flash-rescue script to allow easy updating of u-boot and installation of sysimage-v3 on eMMC. You can try it out yourself with the changes from this merge request: reform-flash-rescue (!14) · Merge requests · Reform / MNT Reform Tools · GitLab
With that, configuring eMMC boot with sysimage-v3 is as simple as flashing the image to the SD-Card, booting from it and then running:
reform-flash-rescue
# answer "y" twice
reform-boot-config --emmc nvme