ffmpeg produces the same output file with different bitrate settings My input is an uncompressed video captured from old Hi8 cassettes, mediainfo:
Format                                   : RGB
Codec ID                                 : V_UNCOMPRESSED
Codec ID/Info                            : Raw uncompressed video frames
Duration                                 : 1 h 32 min
Width                                    : 720 pixels
Height                                   : 576 pixels
Display aspect ratio                     : 5:4
Frame rate mode                          : Constant
Frame rate                               : 25.000 FPS
Standard                                 : PAL

I have looked at the sources I could find and tried to come up with the optimal settings for my encoding needs, while also trying out a few dozen combinations, and the best I could come up with looks like this:
ffmpeg -i in.mkv -vf yadif -c:v hevc_nvenc -preset slow -rc:v vbr_hq -cq:v 19 -b:v 2m -maxrate:v 3m -bufsize:v 1m -c:a copy out.mp4

The output's mediainfo is:
Format                                   : HEVC
Format/Info                              : High Efficiency Video Coding
Format profile                           : Format Range@L3@Main
Codec ID                                 : hev1
Codec ID/Info                            : High Efficiency Video Coding
Duration                                 : 20 s 0 ms
Bit rate                                 : 5 142 kb/s
Width                                    : 720 pixels
Height                                   : 576 pixels
Display aspect ratio                     : 5:4
Frame rate mode                          : Constant
Frame rate                               : 25.000 FPS
Standard                                 : PAL
Chroma subsampling                       : 4:4:4
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.496
Stream size                              : 12.3 MiB (97%)
Codec configuration box                  : hvcC

What I have found is that I can increase the -b:v 2m -maxrate:v 3m -bufsize:v 1m options tenfold, and the output still stays the same. How can this be the case? I am thinking I messed up the settings gravely, in this case, could you please correct me and set me to the right ballpark? First I tried without the bitrate options and only -rc:v vbr_hq -cq:v 19, but that resulted in terrible quality and smaller size, and then I found that I need to set the bitrate as well, unlike just the -crf like I used to do with libx265.
Abandoning -preset slow also doesn't change the result.
I have also tried libx265, which I have used beforehand, but that's a 10-20x speed decrease, so I wouldn't want to use it for this 15+ hour project. One thing I see is that with that the chroma subsamplig is 4:2:2, instead of 4:4:4. Setting it to 4:2:0 actually increases the file size, what should I do with this?
Thank you in advance for any help, and please let me know if I should give more information in order to get the help I need.
With hevc_nvenc, if cq:v is set, b:v is ignored. Change -cq:v in conjunction with -maxrate:v instead.