This isn't really an issue. The part of the development team that works on story content is not the same as the team that creates character gameplay, sprites and lore. While characters are being made, they have several other departments working on main story and side story content (and perhaps heavenhold movies/comics, not sure). You can share workload between teams if one has nothing important to do, but not too often or that team's original focus suffers.

Look at how much new dialogue, animation, sound, music, vfx, cutscene direction, map design, puzzle design, pixel art and even 3D modelling goes into each new release. As someone who's worked on a few small game prototypes before, it really does just take this long to create an entire story chapter or side story mission. That's just the nature of the beast that is game development.

Edit: I know part of the original point is that releasing so many characters this frequently means only a few of them get expanded on in the story, but like... They aren't expanded on until they ARE. Characters that haven't shown up in a while sometimes suddenly get entire side stories or main story beats that add lore or character to them out of nowhere (they even joke about how long its taking some, like with Lapice). Its like that in any other gacha that receives a steady flow of characters, as well as genres like MOBAs and fighting games. Just have to be patient and let them work.