Welcome to the post-covid post-japan-reopening reality. Supply and demand... everyone and their mother has a crapload of miles like you do from the last few years, and unfortunately airlines have all tightened up award availability to capitalize on higher demand coming out of the pandemic, so it's a double whammy... especially for Japan which has demand exacerbated by the long closure. As far as switching goes, I have never been a fan of storing miles/points with a particular carrier - NH or otherwise. I build up MR and UR and transfer accordingly when looking to book.