I have heard many positive things about Yamasa, too.
In my days (that's about 20 to 30 years ago), many foreign nationals (I know) who made great progress in Japanese language learning went through the Japanese language program at ICU (subsite.icu.ac.jp/jlp/index.eng)
There may be other schools in the Tokyo area now, with advances made in the field of Japanese language teaching, but I don't know much here... :emoji_bow: