Well we should state the obvious first. That being that yeah there was a capacity to conduct amphibious landings to some degree as evidenced by their landing elements of 2 different Rifle Corps during 10 days in the middle of August in separate locations supporting a ground advance in Sakhalin and landings in the Kurils.

But these landings were in general characterized by if not amateurism, than having to make do with limited resources combined with limited experience. Relying on capturing port facilities as the first step, running ships aground if needed, and a general lack of the numerous specialized groups and practices in use elsewhere.

But that isnt to say they had no tool to bring to bear that wasnt taking a destroyer or even cargo ship and aiming it at the beach or using small lighters and barges not meant for the task. On top of the gaggle of landing craft, riverine craft, and supply vessels they had on hand, some aid did come by way of Lend-Lease. Under Operation HULA, nearly 200 ships and vessels of all sizes were sent to the USSR. Crews would meet their new ships in Alaska where they would spend a few days or weeks with American crews familiarizing themselves and then depart with other aid convoys and reinforce the Soviet Pacific Fleet. Most of these were small combatants like frigates or minesweepers and corvettes. But also about 2 dozen landing craft made the trip by VJ-Day as well. These were split evenly between LCT's and LCI(L)'s while more of each type, though not many, would have been in the pipeline for delivery. It was a contant point of agitation though Admiral King resented having to send the USSR any craft that might be of use for the USN as it prepared itself for the possible landings in Japan though the Admiral himself had deep reservations about the idea.

So it isnt much but between what the USSR had on hand(older transports and warships, cargo ships, small combatants to load with men, and river craft if weather cooperated), bolstered on the beach with some specialized craft from HULA is isnt a crazy prospect that a landing could have been tried in September of 1945 with at least ok prospects.

While the logistics situation wasnt great there were several factors in their favor. The main ones being that Japanese defenses on Hokkaido were focused to the East and South against a possible US invasion. And the resources needed to reorient were in scarce supply, at least on a timetable of weeks vs months. That was helped by the extreme difficulty of moving any large loads of groups to Hokkaido as US naval and air force strikes had sunk or shot to pieces many of the large ferries used to move to and fro. While range for air support would have been iffy, depending on the beaches chosen it could have been in range of the mainland for Soviet aircraft, including smaller fighters like the Yak-7/9's or strike aircraft like the IL-2. Though it would meant a long open water flight to and from for the crews, which would cause issues should a serious enemy air threat like Kamikaze sorties occur.

So if the war doesnt end in August the pieces are at least in place for a Soviet landing in Western Hokkaido with at least a divisional sized, and more likely Corps sized force getting as many men ashore on some of the few workable beaches, and then trying to capture a functional and usable port to bring in more reinforcements and supplies as they simply didnt have the tools to operate from the beach in large numbers for any length of time.

And in the end that is all that matters to start changing the geopolitical situation post war, that Stalin could point to Soviet troops ashore on the Home Islands.

For a nice list to see all the different amphibious warfare ships transferred to the USSR check out this link: https://www.ibiblio.org/hyperwar/USN/ships/ships-lc.html

The site also contains lists for other small craft like frigates and minesweepers which would be useful for landings too.