The guy voices Eren and Koichi

Erens voice actor voices Koichi?

Yea.

I would honestly choose a different seiyuu than the options given

If I were DP I'd choose Takahiro Sakurai, even though he's already voiced Rohan but it's actually pretty common for a company to have someone voice act for multiple character in the same show.

For the English Johnny Young Bosh.

Whoever voices Eren Yaeger

Who voiced Johnny in EOH/ASB again?

The same guy that voices Eren in AOT

And Koichi