I have both the Dekopon and Shiranui, the they look and taste very similar, but my Shiranui is a dwarf with a ton of flowers.
The Dekopon is a 6 foot tree, mature and still holding fruit, minimal flowers.
I would think they'd flower the same time but they don't.
Taste wise, they've very delicious, sweet, lightly acidic, soft chewy bite, easy peel, easy to chew, no fibers, very easy to eat.
Never seen a single seed, in dozens of fruits I've consumed, very strange didn't even know they had seeds.