I'm from the UK, the original home of the kit kat. Although different flavours are launched periodically it's nothing like the Japanese versions.
It's the same kind of deal as some of the Japanese McDonald offerings - you think you're familiar with the brand because of the ubiquity of MuckyDee restaurants in the U.S. And then you have a McDonalds' ebi(shrimp) burger.
I first visited Japan in 2002 and initially went wild for Japanese kit kats but I reached saturation point some time ago. However, I do have a child who has friends, and still buy bags of kit kats with interesting flavours (particularly matcha green tea) from Japanese drug stores - cheaper than the boxed sets from souvenir stores.
Everywhere you go in Japan, and I mean everywhere, you will be confronted with different kit Kat flavours that represent the region, an event, a season, a just because. Not all are good. I think I got over my initial obsession after eating the Yuzu edition. I LOVE yuzu, it just didn't work as a kit kat.

As for Royce chocolates. Don't get it. Have been offered samples whilst queuing for Shiro Koibito white chocolate cat tongue cookies, but didn't get what the fuss was about. Not a fan of Tokyo Banana either. Yoku Moku biscuits on the other hand.... My love for these will never die.

But don't worry about missing out. Just visit the basements of department stores (depachika) and keep your eye out for samples. Don't be shy about tasting and getting written evidence of what you had so you can keep an account of what you actually tried and might want to buy before you leave. You may not be able to understand the Japanese, but if you have a phone number of the company who make the product, you can google details that way.

Be aware that in March items related to Sakura and/or Girls Day will seem ubiquitous - then many of them will completely vanish. Don't be surprised if you go back to look for things and find they've gone by the end of your trip. Have fun!