I found the ZyXEL NWA50AX which appears to also use NMBM. The node used there is just &nand with a few options. I'm not sure which exactly to set.
Otherwise, I began creating the flash layout based on the OEM layout from @lostinfever's bootlog.
I based this on the AX6S OEM Flash Layout did similar things like squashing down ubi and overlay partitions (like so on the AX6S).
Not sure if this is even right, but also wondering where the Nvram partition goes to if its replaced by u-boot-env.
They don't make this part easy! (and I don't expect it to be). Looking for more feedback from experienced members again!