I am of the unpopular opinion that the spelling of English character names should always be as close to the Japanese pronunciation as English can get, even when they are English loanwords or foreign names. I still remember people insulting me and telling me how stupid I was when, right after she was first introduced, I claimed that Sasuke’s daughter in the Boruto manga should be called “Sarada” and not “Salad”.

I don’t even think Eustachius is a great way to spell the name, despite definitely being the spelling that the Japanese came from. We just don’t pronounce that name like the Japanese pronounce it. Kazuki-sensei has admitted that Damuel was originally a typo of Samuel. Why not just call him “Eustocks”, “Yustocks”, or something similar? The only important thing in a translated name is the pronunciation.