yabukita
I’ve only tried one Japanese black (Wakocha 和紅茶) it is from a 100 year old tree that was a gift from a friend in Tokoname. I posted about it here. How was your flavor profile? Not sure what cultivar mine is from, but it is highly aromatic with notes of aged fruits, and flavors of plum and spices. Very good. I’ll have a session with it in the coming days and try and think of something similar.
The only Japanese black teas I've had were ordered direct from Japan--one from Yuuki-Cha, and one from Obubu. I did not ever figure out a sweet spot for the Yuuki-Cha versions, but the Obubu tea reminded me a lot of some very fine floral and fruity blacks from China (Yunnan gold, Jin Jun Mei); Taiwan Ruby black or Sun-Moon-Lake black. They all share a lack of overt bitterness as well as the fruity notes that are so appealing.
From my limited experience with Japanese hongcha, those would have been my starting point as well. Japanese hongcha has a very clean taste, as opposed to the more rougher Assam-kinds of other regions. Taiwanese Assam can match the cleanliness and in most cases beat it by a length in terms of complexity and sweetness.
That said, a lot of Taiwan hongcha of the Hongyu/Ruby/etc/ kind is heavily sprayed with pest control spray, grown in lower elevation and often harvested in summer, bugs are a concern for farmers. The organic versions of these are not so easy to find.
A safer bet would be high mountain, or honey oolong as they call it here. I usually prefer those over the Assam-hybrid varietals. But it is a different flavour profile and might not be what you are looking for...