I see the source of confusion nonsuch.

You set bitrate with -b:v and -maxrate, instead of using the encoder designer's quality preset (crf). That is an error.

For local playback with both H264 and H265, ffmpeg documentation instructs to use a single pass encode with the crf parameter, not two-pass limited bitrate. With H264 and H265, unlike some prior encoders, two-pass is only superior for encoding to a specific filesize and the maxrate parameter is used to limit bandwidth for streaming.

Your settings waste bits in H264 and H265 during more-compressable scenes, but they hurt H265's efficiency more because it is better at rendering simple, low motion scenes at very low bitrates. The crf algorithm more aggressively varies bitrate than the -b:v one.

In summary: You have not found better encoding for H264 -- you're misusing both of them, but you're degrading H265's efficiency more. By doing-so, your H264 encodes look relatively better vs H265 than they should be.

Hope this helps anyone browsing the thread.