Prores ffmpeg questions
1) your Panasonic footage will be H.264; GoPro may be H.264 or H.265. These are inter-frame codecs whereas ProRes is an intra-frame codec (each frame is coded standalone and doesn't depend on other frames for decoding). So, you should consider a higher bitrate to maintain source quality.

2) FFmpeg does allow you to set a custom quantizer but by default it will use a fixed quality setting per profile. For custom quant, use -global_quality N.

3) FFmpeg has to convert to the given pixel format which is what output -pix_fmt will do. The encoder only encodes i.e. takes in raw uncompressed input frames and delivers coded data.