I think
Caspian is great but
Casper is kind of annoying. And
Caspian would definitely sound better with
Theodora!
I have the exact opposite impression about the modern vs. traditional thing.
Casper fits in more with names like
Carter and
Tanner IMO.
As for the other kid named
Caspian... idk. Do you already have a middle name picked out? Because you could go with
Percival Caspian or
Caspian Percival and call him
Percy. Or use a name like
Cassius,
Cassian,
Casimir,
Crispin, etc.