here is a realstic eye that u are looking for your character.Every part of the eyeball can be changed. for example:U can change the pupil and the iris color by changing the color ramp node in iris material.u can also change the iris colors there.U can change the other parts of the eye by going to sclera material.for changing the vains size,color,white part around the iris and etc.This model can be used in both Eevee and Cycles render engine. U can also bake the textures and export the model to any format u want to use in any app.in ''cycleeye'' u can see the model rendered by cycles render engine.u can change pupil shape too by going to sclera material then in mapping node u can change the shape and the location of it as well.