1) Outer Manchuria needed much investment to exploit its resources, in timber, coal and tin and others.
2) Trans-Siberian Railway passed thru Outer Manchuria. If the land would be ceded to Japan, what would the situation and the ownership of the railway be?
3) Japan as an upcoming regional power needed resources as early as possible. So an British or French Alliance would help.
Please note that the famed Russian short story author Anton Chekhov recorded his travel experience in 1890 -- mere 15 years before the War -- in Sakhalin and Vladivostok. Given the living and development conditions of both places, they would not have changed significantly in 1905 to 1906. This realization supports my point #1. G
In the light of the 3 points above, the Outer Manchuria and the Railway ownership and Chekhov's experiences, it is recommeded Japan would have owned the whole Sakhalin island, thus securing its crude oil and lumber resources, plus the right to timber, coal and tin mining in OM with Russia. However, the Russian side during the talk for Treaty of Portsmouth insisted not to cede the whole island, if I recalled correctly. For simple transportation on the island, immigrant could use wood gas in the internal combustion engine of cars and trucks. Sawdust and pellets from lumber and paper mills leftover materials derive the wood gas.
Given the Japanese governance was more proactive in improving the Southern part of the island than its Russian and Soviet Union counterpart in the north in the OTL in early 20th centuries, in an ATL Japanese lumber and paper companies with government subsidies installed wood gas refuel stations on the side of a road. Paper and timber mills sent leftover wood materials to the refuel stations where they would be ground and processed to produce wood gas on demand. The wood gas was burned to supply its neigborhood energy and would refill the gas cylinder on a truck. The disadvantage is the slow start up speed, usually not an issue for civilian use. Military vehicles would still use liquid and gas derived from crude oil.
Then this ATL would be how Japan would win more trophies.
In OTL, Japan occupied northern part of the island and some communities in OM in 1920s. So winning more lands after the war would cease the incentives to take over those place as in OTL, given the ever watchful American eye.