I have a SBC based on another Rockchip, and the last time I tested it, it was pretty useless as a media player in all regards.
H264 video stuttered, HEVC video would lock up after 10 seconds of playback, no HDR support, etc.
Please try the latest LibreELEC 9.0.1 / Rockchip Alpha 014 build (on a rockchip sbc with a "supported"/tested devicetree), it should work very good.
Do not stare blind at the alpha name, it is mostly there because there is features missing (HD-audio and full HDR infoframe metadata).
So LE has stopped supporting Amlogic and Rockchip users?
Rockchip is very much working and "supported" even if it is still flagged as alpha, the 9.0.0 / Alpha 013 and 9.0.1 / Alpha 014 images should work very well.
My focus for Rockchip at the moment is on mainline linux, hd-audio and adding more codecs to the rk v4l2 request api driver and ffmpeg hwaccel.
I still don't understand why you fixate on the Kernel version, how old it is, or what GPU drivers it has. As long as it works well and fast, and it does work well, and it's also very fast, I personally don't care what version of kernel it is running on.
LE pushes towards Mainline kernel support, I can understand why, LE has to cover multiple different platforms, so it makes sense for you.
Even with the 4.9 aml kernel the drm/kms stack is unfortunately not used for video rendering. Compared to mainline (or RK 4.4) this mean that the new video rendering path in kodi (the direct-to-plane drmprime renderer) cannot be used. When amlcodec (and mmal) rendering is removed from kodi the kernel subsystems used for the display stack gets important.
The drm prime decoder + direct-to-plane renderer was first designed and tested for RK 4.4 kernel but thanks to using "modern" kernel subsystems it is working with any drm/kms platform that can render video on a dedicated video layer / plane and has been proven to work very good on mainline aml, rk, rpi, imx and other platforms.