50GB BD-R will work, but you’ll probably want to scale back the audio tracks, or you’ll risk going over the total bitrate limit for 4K Blu-ray players. I did just one lossless track and several lossy, and that was fine. You could probably be less conservative.
I assumed that only the active audio track contributed to the total bitrate limit. Am I wrong about that?
IIRC, the player reads all muxed tracks simultaneously, then decides which ones to play. So you can go over the limit even when the tracks you’re actually using don’t go over that limit.