Ooh I don’t think they’re really unique. With a name like Elias as well, I wouldn’t just look at where it is on the charts but I’d also count where Elijah and Eli are. I know they’re different names, but it’s like Emilia/Amelia/Amelie. All those parents think oh number whatever, that’s not bad, but it feels sooo much more popular because there’s also so many extremely similar (basically the same) names. So if you’re going for not popular I’d totally avoid an Eli - I know loads from babies to my oldest son’s age. I’ve never met a Caleb though.
Maybe Cyrus or Silas? Similar sound to Elias. Or there is a website, can’t remember the name, where you type in names you like and it gives you a long list of similar type of names. You could use something like that to find a similar but less popular name.