Black Sea events show the risk. Just ask the Russians. No idea about the Kure base but I still think too many eggs in the basket sucks.
Black Sea is totally different. It's 400,000 sq km of open water, with Ukraine having a coastline on it. Russia has no control over access to it. Ukrainians can launch USVs into it directly from their own coast. They can also fire missiles directly, as when they sank Moskva, & launch drones directly, to fly over open water & attack Russian ports.
The Sea of Azov is surrounded by Russian-controlled territory & has a narrow entrance, but Ukrainian-controlled territory isn't much more than 100km away. Hitting ships in it isn't difficult. Ukraine's done it mostly with drones, I think.
The Seto Sea is just 23000 sq km, with three narrow entrances & many islands. There are bridges across it, via islands. I've seen the biggest. To get to Kure one has to pass through at least two narrow passages, either the Kanmon strait between Honshu & Kyushu (<1km in places, bridged, & very busy, with cities on both sides), or, more realistically, the Hōyo Strait (14 km & deep), & then one of a few narrow channels through a chain of islands. The biggest one is ca. 3km, & is the main shipping channel. Approaching from the east is worse: under three bridges.
The JMSDF has proven itself to be rather good at tracking submarines. Chinese submarines have tried to sneak through some of the channels between Ryukyu islands & been tracked all the way. Japan has a sophisticated SOSUS system.
Missiles or drones would have to fly a long way including over at least one of the Japanese main islands, through Japan's air defence system.