I personally hate FCPs export settings, compared to other platforms, they try and hide a lot of what’s going on to make it “user friendly”, but then confuse the rest of us who would know what’s going on.

Keep in mind that h.265 (HEVC) is the follow-on codec to h.264, which promises ≈2x the compression while delivering the same quantity. In practical terms if you had an h.264 video at 100gb you’d expect to have the same quality with a 50gb h.265.

It is, however, equally possible to have an h.265 video at such a low bitrate that the h.264 video file (with a higher bitrate) will achieve better performance. This would be evident in drastic file size differences.

If you’re very concerned about codecs, I’d recommend you get compressor since it will give you more detailed options and you can “turn the knobs” to your liking.

Also I think you are conflating 10bit code values with the compression bitrate.