I will not speculate on what an invasion would have been like, I don't like what-if-history. I can however tell you a little bit about the Japanese preparations for the invasion of mainland Japan.

First of all however, let us establish a few basic things. War is a pissing contest of national economy. Japan was ruined, financially and industrially it simply lacked the resources to wage war by 1945. The Japanese army was still formidable in 1945, however, it was largely spread out over south east asia and china, relatively few units were in mainland Japan and those that were primarily consisted of garrison troops and raw recruits. Most well trained and equipped units had over the course of the pacific campaign been shipped out of Kwantung and mainland Japan, the units that were left were of relatively low quality compared to what the US forces had faced earlier in the war.

The plan to defend mainland Japan was to stop the allies at the beaches. The defense of Kyūshū was intended to hit the marines coming ashore with everything. 3000 kamikaze planes, 5000 suicide vests and just about every unit Japan could spare.

Would this have worked? Probably not. Kamikaze planes were never very effective, particularly not towards the end of the war when the allies had learnt to deal with it. Banzaii charges were never very effective either, I can't imagine widespread use of suicide vests would have improved them by much.

This argument is often used by people to argue that the Atomic bombs were excessive. However, do keep in mind that the US did not have a very accurate picture of the defenses of mainland Japan at the time and that the last two battles they had fought (Iwo Jima and Okinawa) had been very bloody and hard fought. It was not unreasonable at the time to expect that mainland Japan would be even worse.