In terms of interest, I think Isaac and Isaiah are pretty equal. I much prefer warm, friendly Isaac, though.
I think the problem is James, tbh. I love it, but it feels like a filler in many combos, particularly when paired with a popular/common names like Isaac. I read below how James is a family name, so I get why you wouldn't want to drop it, but have you ever considered rearranging some of your combos in order to pair James with a less common first name and Isaac with a more uncommon middle? Something like Isaac Wolfgang and Sebastian James, perhaps, or Isaac Tempest and Edmund James.