So, basically you're saying that the /boot/config.txt file does not exist, and that /boot/armbianEnv.txt has to be used instead.
Unfortunately, it seems that an overview of all possible parameters for armbianEnv.txt does not exist, as it depends on the kernel version.
Anyway, as per the Kernel Parameters page for the kernel version I'm on, I believe the boot_delay parameter is not what I need, as it will delay each printk during boot.
I just need to add a pause of 30 seconds at boot... before the network is brought up.