Sanctions are not collective punishment

Not according to the state department. They themselves say the reason they use sanctions is to cause as much pain to the people as possible so they're more likely to overthrow the gov't, or failing that, be softer targets for military intervention.

DPRK did invade the RoK

The RoK was doing genocide and had been recognized as the gov't of all of Korea in the UN due to the US's machinations, a gov't whose election was rigged in the south, and absent in the north. The elections the north arranged were of course, ignored.

The DPRK saw that their position was unsustainable and struck while they still stood a chance. The war started when the people of the DPRK were faced with an existential threat.

Which is kind of similar to Russia's invasion of Ukraine, but nobody here is gonna pretend Russia are the good guys, just that they're fighting a greater evil.

Also also, China has reserved a spot on its equivalent of the National Mall for when it takes Taiwan back.

Do you think the RoC can feasibly remain independent forever?

Eventually either PRC is gonna be able to make them a better deal than what the failing American empire can, or they'll make long-term peaceful integration infeasible, necessitating short-term, violent integration.