Greetings Marsendra,
Our guild, The Twilight Tier (Horde, Saurfang) might be a good place for you to hang out.
As a player with a history of on-and-off play, I too have found myself going through content solo, without a guild. And I agree with you, it does get lonely. It’s one of the main reasons we brought our guild back from inactivity. After all, the game is much more fun with friends and like-minded players.
If I’m not mistaken the time difference between EU servers and Japan is 8 hours (GMT+9?). Our active times are generally in the evenings for us (and weekends), but there are a couple of members who do play throughout the day, so chances are there’ll be a few players online during your active times.
If you’re interested, send me a DM on Discord (Veletala#8483) or Battle.Net (Veletala#2216) and we can chat further.
Best of luck in your search for a guild you can call home!
~Veletala