How to get best stream quality with limited raspivid bitrate and fps?

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I’m using RPI3b to push CSI camera to rtmp stream, cmd:

raspivid -t 0 -w ${w} -h ${h} -fps 25 -b ${video_bitrate} -roi 0.25,0.0,0.75,1.0 -o - | \
    ffmpeg -thread_queue_size 512 -i - \
    -c:v copy \
    -f flv "${up_pushurl}"

RPI seems using hardware encoding, resulting of low cpu and memory usage，good. And bitrate are limited as cmd.
but for better resolution, 1080p, e.g., will stream quality decrease?
manual shows –level(4~4.2), -qp(10~40), what’s constrains between bitrate and quality, level and qp parameter?
I want to get best quality with limited bitrate (<=3Mbps) and 25 fps

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