Very low bitrate/quality on ffmpeg + rkmpp_h264 encoder

hey there, finally managed to have the time to build the ffmpeg from https://github.com/jjm2473/ffmpeg-rk with support for rkmpp h264 de and encoders in hardware. I did notice that the encoder however seems to ignore the profile setting(the only setting it really takes) and encoding by quality doesnt seem to be used either. If I just do a ffmpeg -i in.mkv -c:v h264 output.mkv I get an extremely bad quality file with about 300-500kbit/s bitrate.(talking full hd input here, so 1920x1080) Even when I manually specify the bitrate, the encoder seems to cut out at around 2.2mbit/s, even if i set a much higher target bitrate. the quality of the resulting file is still bad. since those things get passed on directly to the rockchip-mpp library(I am using a current pull from the rockchip-linux github repo since the debian one from the radxa repositories seems to be too old and doesnt allow for a working ffmpeg build), I guess here is the right place to ask.
Apart from that, I am aware that rkmpp isnt really supported with rk3588/the rock 5b and the official way seems to be to use gstreamer. However I have no real prior knowledge of gstreamer, especially not on how to use it (with the hardware dec/enc). Is there any tutorial on setting that up on the rock 5 b (I think I got the setup part in terms of installing everything needed so far) for transcoding of video files?

Apart from that, any workarounds that will make rkmpp on ffmpeg either produce files at the requested bitrate or allow for more settings to get at least watchable files out of it?

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