You changed the format from grayscale (8 bit colordepth) to RBG with 32 bit colordepth. That was unnecessary. You just blew up the filesize for no reason. Your black doesn't get blacker, your white doesn't get whiter and your image doesn't get sharper if you save your BW images with more colordepth. If you adjusted the colors correctly you can't see a difference between RGB and BW without looking on the filestats.
Now to the good things. Good job on the leveling. And saving your images as pngs instead of jpgs, good choice to avoid further quality loss with RAWs from that source.
The majority of BW manga can be 16-color BW PNG and be drastically smaller in file size than a color JPG. Hell, some scans can get away with 8 colors. Just a tip.