I thought the same since his older brother was very cocky and got killed very early on. I think they mentioned way back then that he was more talent despite being younger, but because we hadn't seen him for so long, I thought he wasn't show up again, so his intro was just so surprising. That was a great "natural" time-skip for him.
Agreed. the night fight (and earlier skirmishes) against Seijuro is up there too. Before he died, I thought most of the stronger enemies were going to survive and be reccuring antagonists lmafoo.
Musashi's fight against 70 samurai is also one of the 3 best fights in the series, along with Musashi's duels against Kohei and Inshun imho.