An invasion by US forces was planned, with expected US losses to dwarf those on Normandy by an order of magnitude. Japanese losses would dwarf those, in turn.

The invasion likely would have been stalled, and the alternate plan was blockade of the islands to interdict food supplies, and Hirohito is on the record after the war as saying he feared a Lord of the Flies-style mass breakdown of society after famine.

But the reason an invasion was planned rather than waiting for starvation was that Stalin was planning to invade. I.e, if the US had been OK with Japan's ending up in the Soviet orbit, both a nuclear attack and an invasion could have been avoided.

There are all sorts of reasons why Hokkaido or all of Japan ending up in the Soviet orbit would have been a Very Bad Thing.

Im not sure Japan would have surrendered. The real question is: why was total surrender the only acceptable outcome to the FDR admin? To the point that mass killing civilians was preferred over a negotiated peace.