Hi JJ -
Yeah! Old Japanese cards are great!
There are, indeed, a lot of unlisted sets (and some sets are listed in the guide but are incomplete, so you may yet find unlisted cards from otherwise catalogued sets).
Menkos and bromides are both available, albeit not in the quantities or with the selection that American cards from the same era are. I know that there are people who try to complete sets, but that seems to me like it would be extremely challenging (especially if you're not actually in Japan).
Here is a
long thread that is mostly about my effort to build a collection of Japanese hall of famers.
I'm happy to talk about Japanese cards, or Japanese baseball, if you're interested or have questions. We have a few members who are more knowledgeable than I am about it, and who can actually read Japanese (to call my own skills with Japanese "rusty" would be to insult rust), and hopefully they'll chime in here.