Rockchip has released bsp kernel 5.10 for rk356x and rk3588. Since multimedia support is not fully supported by mainline kernel, a legacy kernel would be good if you want h264/hevc hardware decode.
To make kodi support hardware decoding, you need:
1, ffmpeg supporting rkmpp. I've packaged it in my ppa: https://launchpad.net/~liujianfeng1994/+archive/ubuntu/rockchip-multimedia, which is based on the patch from https://github.com/JeffyCN/FFmpeg/.
2, kodi built with gles render syste