Japan was never all that democratic, not in the currently understood use of the term.
The Constitution was designed, from the start, to give both the Army and Navy an absolute veto on the rest of the government. Both the Army and Navy Ministers had to be seated before any government could be formed .Both HAD to be active duty officers, meaning that either service could collapse the government at will. It was effectively a military dictatorship with a human face.
What happened in the late 20's was a rise in Nationalism making the reintroduction of a perversion of Bushido almost inevitable.
But to address the two specific colonies.
Formosa was far more likely to become a permanent, more or less willing part of the Empire (there are some similarities to Hawaii, there is a small political movement pushing for native Hawaiian rights, but the number of people who actually are pushing for independence amount to a handful (in no small part due to massive migration from the Mainland, Japan, and the Philippines over the last 120 years). The dominant "local"population of Formosa had little to no identification with Mainland China something that would allow a reasonably gentle colonial administration to succeed.
Korea, to the contrary, had a long established national identity including a fabled history of defeating Japanese aggression going back to before the establishment of the Tokugawa Shogunate. The population resisted, to a strong degree efforts in "Japanization". This effort, which was remarkably heavy handed, up to and including requiring the changing of everyone's NAME (in some cases TWICE*), suppression/outlawing of the Korean language while attempting to hijack parts of Korean culture to make it appear to be of Japanese origin.
If Formosa can be likened to 1970 Hawaii, Occupied Korea can be best likened to 1916-23 Ireland, which continues to have a serious flashpoint to this day with the "Six Counties"/Northern Ireland despite a vastly more "enlightened" administration out of London.
*
https://en.wikipedia.org/wiki/Sōshi-kaimei
tl;dr: Formosa okay, Korea sucking chest wound