Also i always used a standard dtb with an additional grub config line:
@Virnik feel free to ask a questions, i’ll try to answer best i can
I would like to, but I did the same thing, and in my case, GRUB complains about improper zImage format. Dunno what I am doing differently. u-Boot boot works fine, both EMMC, SD or ZFS/SSD. I also tried netboot, and it also works fine. GRUB also boots. But if I’ll select my kernel/OS, it tries to extract the image, and then complain about its format.
As for DTB, I also used default dtb file, like you.
I’ve been on Gentoo some ~20 years ago. Since then, I become lazy, and ended up enjoying (K)Ubuntu. But I am quite fluent when it comes to rebuilding various SW. As for embedded, I am not new, but lets say I slept for past 15 years or so. That is how my old RouterStation and RouterStation Pro boxes are old. Since then, I didn’t build any other tailored distro for any board. And while my build works, I still do face some cosmetic issues, like this one. I’d prefer GRUB over u-Boot, but in my case it doesn’t work. I’ve made both temp/hum/press sensor work, Gas sensor too (ADC+PCF8591), Wireless regdomain also no longer stand in a way, frr is also setup…but this goddamn thing simply do not work as I would expect it to. I have to be missing something, doing some mistake somewhere, but I am not certain on where.