My knowledge of Japanese grammar is in Japanese, so I'm not certain about the English term of 主語, to be honest. We use the same term to describe 'subject' in English grammar. I used 'topic' just because the original article used it.
Your explanation of 'ga'/'wa' is spot on as far as I can understand as a layman of native speaker with standard Japanese grammar education in Japan but no advanced linguistic degree.
I'd say that, because 'wa' emphasizes the introduced subject as the center of interest, it isn't used in the subordinate clause.
Tanaka ga nihon ni itta hi. (The day Tanaka went to Japan) ; ok - the interest is on 'hi'
Your explanation of 'ga'/'wa' is spot on as far as I can understand as a layman of native speaker with standard Japanese grammar education in Japan but no advanced linguistic degree.
I'd say that, because 'wa' emphasizes the introduced subject as the center of interest, it isn't used in the subordinate clause.
Tanaka ga nihon ni itta hi. (The day Tanaka went to Japan) ; ok - the interest is on 'hi'
Tanaka wa nihon ni itta hi. ; invalid