My two cents is that not all NPCs are created equally. The purpose behind an NPC determines what that NPC needs to make them impactful. An NPC with multiple purposes needs that much more work, but more work makes for a more vibrant character (usually).
However, you have to know when making them that they may not be used for whatever purpose you design them for. Players will interact with them as they choose and things generally spiral off into crazy directions. Sometimes we spend a lot of time and effort into making NPCs then get really disappointed when players ignore/dismiss/misinterpret them.
Anchoring a character to your game world is an easy way to make sure that your NPC gets the attention it deserves. Making them important enough that the world doesn’t work right without them is a surefire way to anchor them. This is easily done at the start of the game. It is much harder to do after the fact.
It is important to remember that every institution exists for a reason ICly. Too many times we create institutions and NPCs for those institutions just because the book says it exists. That waters down any NPC if they’re only around because the theme says they should be around, but we ignore the reason behind it. An impactful NPC changes the world around them not only with their presence, but with their absence.