They have Different teams working on them. They likely have at least 3 teams. ! for events (easy Job) 1 for New packs (D*** Heads JK) and 1 for next World (really hard job). This takes a long time as they need to balance new character skills create the art and balance the new mechanic by only Implementing certain cards. Making the scenery doesn't take long but the standard and Legendary decks do.
What did you say? WHAT?? GX is so much better than 5ds. At the start, GX had a lot of meta decks like Cyber Angel and Ancient Gear, but 5ds didn't change anything in meta, don't say blue eyes because it's Seto's deck. Even in themes just hear Yubel's theme and Alexis's theme, can you compare it with Akiza's 🔥 theme? Any comparison you're talking about? Let's see if 5ds's new char can do.
The heck..
While you mention Yugioh, you also imply the name of yugi which is the character of DM. LOL....
Also all the movie revolves around DM World (except beyond time DM,GX,5D)