Personally, I've never held that forceful change of culture = genocide. Normal genocide is death/rape in near total (or the attempt of it). Cultural genocide is thus the complete eradication of the previous culture, and everything it stood for. So yes, the humanity declaration was bad. But it wasn't linked with a thousand other things to purposefully kill off the culture such as banning Taoism & Buddhism, especially Zen Buddhism, the destruction of shinto shrines, forceful movement outside of the main islands, etc. You'd need a lot of stuff to make it qualify. Basically there's a really high bar to meet here. Kidnapping and reeducation meets it (or at least meets attempting it).
Notably, kidnapping and reeducation has been tried in the past (notably the US government and native Americans), and I'd say that comes pretty close to qualifying as an attempt.
Apologies for the lack of a precise definition, but I think to start let's establish some examples of what we can all agree are and aren't cultural genocides.
I'd say classic examples are the CCP in Tibet and with the Uighurs, US with the Native Americans when it came to the Indian schools, and various medieval powers forcibly converting Jews.
Non-examples would be the Brits conquering India, US treatment of black citizens post civil war (awful, not genocide though), and the example you just gave with Japan.