A character voice is not just how they speak, it is also what they say. Use dialogue not as only a method to advance the plot, but what characters are saying and how, should help inform the reader on who they are . A kind character will never say something to offend another, and so on.
You mentioned the sentence structure and choice of word, but you could also use what they say and on what they focus on when they speak as a source of character voice. For example, the character who speaks formally, are they more of an artist, or someone more analytical. The answer to that question will help inform when they speak how they describe things.
You could also add text explaining what they’re doing while speaking and the tone they’re using as a method characterisation. Are they looking at the person they’re speaking to or are they doing something else. Is their voice monotone or filled with excitement? Adding descriptors when the character speaks should help.
The other thing you should accept is that sometimes you're not going to succeed at using all the methods of creating a character voice for every line of dialogue. Sometimes the need of the story will take priority, but as long as what is said doesn’t stray too far from what is established you should be okay.
Hope what I said made sense, and helped.