You know it is interesting that despite not having cultural influences from other countries such as America we still have a cultural sphere that is almost the exact same from modern Japan except for a key difference being that the supernatural is a piece of public knowledge. Despite everything I've done it still isn't that hugely different as we enter the 21st century level of technological advancement if anything my western origin and influence on the people are what pushed the country back into the same societal state as mundane earth with it being more ingrained into the culture rather than a sort of novelty from overseas.
As for knowledge of the supernatural I've made it so that there isn't a lot of secrecy involved with magus as knowledge of who is magus is not so much public knowledge but it isn't a secret either. That is mainly because the government tries to manage the magus not so much on the level of making micromanaging all magus but more of a set of well-defined laws that make it so child bug rape can't happen. On the other hand, they also promote the study of magic in order to make society better and Japan safer from things like a phantasmal beast with benefits and prestige especially in the political world.
We also have a government agency that is established from magus called Enforcers to police magus should anyone start to stray from the righteous path. Also, it would kinda be hard to hide magus and the supernatural anyways with the fact that as we now enter the digital age with cameras and pop culture beginning to get defined we see lots of shit online like videos of some magus that like to show off to look cool or videos of Enforcers hunting phantasmal beast.
It is now -2000 BC/BCE we have the technological standards of 2020 AC/CE of mundane earth with the Japanese calendar reading the year 2,000 URE (Uchiha Reign Established(Basically 1 was the year I took over and established Tokyo (Author couldn't think of anything better))) we had a beautiful new year without the whole Y2k fear since we were already ahead of the curve thank the Source I didn't have to deal with that.
What I did have to deal with though was nearly having Korea be America to Japan's Britain but thankfully I have people who are better at dealing with politics than I am. The problem I couldn't delegate to other more qualified people is dealing with weak gods from small populations pestering us but I'm strong enough to make them back off from my land. I'm like the closest thing to being a Divine Spirit without having divinity through birth. The gods themselves were random and forgettable as none of the real big hitters have shown up on the scene yet like the Greeks or Yahweh (Judeo Christianity God, IIRC). They asked for our tech, resources, and people while my people declined with the knowledge that I would protect them then they got angry then I showed them they are very mortal before me and any of my anti divine projections.
Then again I have zero ideas if I might have actually become a human god because and I'm probably butchering the actual explanation of DC history but I might have inherited a seed of divinity from Ragnorok event that also created new Genesis and Apokolips. So fuck if I know how this universe works since it is a crossover meaning knowledge of either world could prove to be unreliable as there is more than just the Greek pantheon that DC. I'm pretty sure only a handful of gods will get seeds of divinity while the rest will remain as pure spirits that fade away to the reverse side of the world. I can't say the age of gods will end but I can't call it the age of gods either after a certain point so maybe the age of mystery seeing as how even if the gods disappear there is a lot of mystical shit still running amok from the endless, cosmic entities, space aliens, and actual DC brand magic. I don't believe for a second that the speed force isn't on some level mystical considering that it is sentient considering it has talked to the flash and is like part of the human unconsciousness(I think?).
Anyways there is a lot I don't know about how the two worlds will mix especially because of space alien-related events like the New Gods and the ultimate Ones. Oh god, I just remembered Type Mercery should be in fucking South America already for like 3,000 years as of now. Thankfully my wish for noble phantasms of the throne of heroes also included god killing weapons of Notes not that I've ever read the Fucking thing so I will have to look for them in UBW and remember them in case of having to deal with them.
God this is getting really high concept for me I don't even want to begin thinking about the moon. I'd rather focus on practical things that I can actually deal with because I don't lack foundational knowledge required to tackle them for example buying really good recording equipment from the system so I can document critical and important events in history both mythological and mundane. Why I'm not particularly sure myself. Mainly my problem is that I've still got 3,900 years till I meet anyone from DC not counting any events of time travel like the caveman batman but I can't really predict those. I could detail how I found and made Tamamo my wife but obviously the answer is I'm the MC so I get the author's waifus in my harem automatically as though this were a slice of life anime.
So I've also started introducing new technological advancements from the DC universe, Halo, Detroit become human, watchdogs, star wars, and fallout. Now it is just a matter of giving them small pieces at a time while advancing society within our territory of Korea and Japan may be set up some satellites and wait for the greeks to show up in about 500 years from now.