I recommend that you try the latest versions of Buster-legacy-desktop Armbian + media script for Firefly Station P1 (rk3399) \ M1 (rk3328). In it, KODI and Rockchip Gst Player works with HW without any containers (including 4k). Details can be seen at the end of these topics.
Thank you for your interest in this matter, balbes150. It seems very promising, but I can only hope it works with your patches, since I had already installed another Armbian Buster-legacy solution and although HW acceleration worked smoothly neither the HDMI-CEC nor reboot worked properly with this 4.4 legacy kernels (to be able to reboot I had to unplug and plug again the power).
I'll download and test it and let you know if it worked. Thank you again.