I'm still unable to build the module correctly. The ZFS kernel module is build for kernel '4.4.213-rockchip64' (the headers version name) but the kernel is '4.4.213-rk3399'.
spl: version magic '4.4.213-rockchip64 SMP mod_unload aarch64' should be '4.4.213-rk3399 SMP mod_unload aarch64'
I'm not sure this can be fixed without editing kernel headers, either :
@ShadowDance Are you using the "current" 5.9 kernel ? Maybe this has been fixed for this kernel ?
BTW as the procedure is a bit tiring to do by hand, I wrote some scripts to automate this (build module on Ubuntu, utils on Buster) : https://github.com/Salamandar/armbian_zfs_compiler/
MOD EDIT: Above is third party script, as always, make sure you understand what is going on before running it on your system.
Having gotten that disclaimer out of the way, thanks for your contribution, @Salamandar!
I took the liberty of marking this as solution. I also made note of the link to refer others to, later.