So right now they use CRF which is generally the best bitrate control mode for h264/h265. The problem is you have no guarantee the bitrate conforms to blu-ray standards which are quite strict. Topaz also is using x264 as the transcoder. I checked with mediainfo on a test output and they aren’t setting blu-ray compatibility
nr=0 / decimate=1 / interlaced=0 / **bluray_compat=0** / constrained_intra=0 / bframes=0 / weightp=0 / keyint=12 / keyint_min=1 /
The audio is also transcoded as 96kbit aac-lc. Which is on the low end of quality for AAC, probably sounds like a160kbit mp3. It also transcodes the audio regardless of the original format which could reduce the quality.
I think it would be better to output as prores and then transcode with a blu-ray preset on something like premiere or x264 manually.