I don't think there is a need to change the names! Especially if it's something that's been growing with you for a long time. Even if you tried to change them, I doubt you would find something that fits your character as much as the original name you gave them.
I ran into the same issue with my comic as I named my character at a really young age, and as a result her name sounds misplaced in comparison to my comic. The solution I found to this was consistency. If her name is strange, then I made sure everyone else had a strange name. This made it so her name didn't stick out or become cringy.
For your story, since your characters have Japanese based names, regardless of the fantasy world, just give the supporting and minor characters Japanese names as well.
That's one suggestion, don't feel pressured to go with it. In the end, they're your characters. Do with them what you want!
Hope this helps