I think this might have been something I missed...
Since the 19th of April, various Nintendo Switch games including Splatoon 3 have been on sale in Japan!
Right now, Splatoon 3 is 30% off on the Japanese eShop, going from 6500¥ to 4550¥ as part of a sale (not confident in the translation of the sales name, sorry). With purchasing the game, it also looks like you'll be able to grab Nintendo Switch Online for free for 3 months.
The sale ends on Tuesday at 12am JST (Monday @ 4pm BST conversion).