in 1945 Soviet amphibious forces consisted of 30 of these
That the USN gave them. The LCI(L) carries up to 200 men up to about 200 miles. No space for tanks or anything as you can see
Nobody is conquering Japan with 30 LCI, especially as the Soviets lost 5 of them to shore based guns when taking the Kurils. The notion of the Soviets being an invasion threat to Japan is absurd
EDIT-
To offer perspective, the US invasion force required nearly 1,400 Landing ships. Not Landing craft like these, Landing Ships displacing at least 20 to 30 times more than these things, often a lot more.
Not to mention the largest armada of warships in history to support them, and thousands of aircraft stationed on nearby Islands. Compare that to less than 200 Destroyer escorts and corvettes the Soviets had available. No carriers, no battleships, nothing.
The Soviets had a hell of an army, but a few thousand guys with no air support, no naval support, no tanks, no artillery, no food... vs all of Northern Japan? The US was expecting a slaughter with all they had on the table, the Soviets? Maybe by the mid 1950s