A while back, someone did a post using Photoshop to convert Manga to B/W. Here is the topic:
https://www.mobileread.com/forums/sho...d.php?t=103583
Also, while searching, I ran into this topic explaining a program, MangAI:
https://www.mobileread.com/forums/sho...d.php?t=157653
If you are not scared of the command line, and lots and lots of reading/learning/testing, I would recommend using ImageMagick to batch convert images:
https://www.imagemagick.org/script/index.php