I am bringing up World War One, yes, because the point was made as if the iron cross is somehow more benign than the rising sun.

The Japan of WW1 used the exact same flag and symbols while fighting for the allies as it did in the second war and continues to use today, while again being aligned with those same allies who are still using their symbols. They are enduring symbols of their nation, and I imagine it simply doesn't strike the Japanese as a necessity to change them out of remorse for an imperial past, any more than the British would consider dropping the union jack or America the stars and stripes.

To your second point, there is absolutely something China could do about it, as evidenced by Korea a few years back requesting that Japan not fly the naval ensign on their ships during a naval review, and Japan withdrawing from the review. The issue over the flag seems to be a uniquely Korean point of contention given that Japanese ships regularly call on ports throughout their former victims in the Pacific without issue, but if countries feel strongly about it the precedent is there refuse entry to Japanese ships bearing the flag.

The iron cross killed millions of people in the First World War and a slightly different looking version of it killed millions more in the second, I'm not sure where the angst about a centuries old motif comes from with the rising sun.

Before the current state of heightened tensions I recall that China had absolutely no problem with inviting Japanese ships to port calls and naval reviews in China, and allowing them to fly their naval ensign. If it was really as big an issue as reddit seems to make it out to be, I'd think it would spark more outcry at the national level.

This is about right. I think the key difference is that the Nazi flag was the flag of an explicit political party, whereas Japan has been using the same national flag since the Meji era, and the rising sun motif has been on flags, in art and on clothing in Japan since the 1600s at the earliest.

In a similar vein to another poster, Germany still has eagles for national motifs and paints Iron Crosses on their planes and tanks, as they've been a part of German martial culture since long before the Nazis came about.

Man loves his plants, simple as