The Reincarnated Prince and the Haloed Sage (Volume 3) by Nobiru Kusunoki

Ryoko Hayakawa, now reincarnated as Prince Herscherik of Gracis Kingdom, is about to turn seven years old. He has no athletic skills, no magic, and can’t even claim to be a great beauty among the royal family, but he still can’t afford to keep playing the subservient prince for much longer. Now that he’s begun his grass-roots efforts to fix his kingdom (just like the period drama heroes Ryoko used to watch on TV), he is met with a mind-blowing proposal from the Minister Barbosse. Meanwhile, the nefarious agents of the Church are on the move, and a beautiful and mysterious woman appears to Herscherik. As he plunges into danger to save his kingdom, how far will Herscherik be willing to go?