Hey @alotje
Could you try again with the latest Armbian Buster current image (with Linux 5.3.x)?
It worked for me when booting from Radxa eMMC + Armbian SD card. Only needed to change /boot/armbianEnv.txt and add fdtfile=rockchip/rk3399-rock-pi-4.dtb as specified in the other thread. First boot can take a bit of time though!