So.
Shmuplations was a site that was originally aimed at collecting interviews with shooter developers, but eventually.expanded to the collecting if interviews for all kinds of games. And they have five interviews with developers of the Phantasy Star series, ranging from 1989 to 2008. Here are links for you all.
Phantasy Star II Developer interview (1989)
Phantasy Star II Developer interview (1993)
Phantasy star I Developer interview (1993)
Phantasy Star IV Developer interviews (1993)
Phantasy Star composer retrospective (2008)