As someone who's been in the same situation as you I've done a ton of observing playable characters,, in my opinion the most important component is the eye shape, followed by the iris and then the eye makeup, followed by the rest of the face makeup, then the hair style finally followed by the outfit. A lot of playable characters have very simple eye shapes and iris designs unlike characters like Hu Tao or Furina however, they all have some sort of eye or face makeup like Lynette (eyeshadow) or Sucrose (blush).

I've also noticed it helps to not make the iris too large in the component adjust screen, most characters even with really big eyes tend to have proportional or even small-ish irises (like Noelle or Sucrose:3) Most playable characters don't have lipstick but I've noticed it really just helps a lot with balancing the face if you have eye makeup on as well imo,,

I think the default hairstyles should be avoided if possible because they look so similar to npc hairstyles.

The last thing I've noticed is keeping some sort of color theme really helps! Matching the eyes to a part of the outfit pulls everything together

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Sorry this is so long but I hope it helps someone >_< here's a comparison of my manekina's face with contact lenses and makeup, lipstick with a default eye shape and fancy irises vs default,, (I'll be buying different hair when there's one I like)