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

Question

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