I like to try and save as many of the 18 as possible while pulling back to Europe as necessary. I try to keep their destination flexible…if Germany is kicking a** they will head to Moscow, otherwise they can stay in Buryatia/Yakut if Japan is not threatening the stack. The stack can also safely reach Kansu in western China by R5.
Generally, I agree with the OP’s initial strategy. Most important thing is to realize that any units moved to Sakha on R1 will die on J2 if the Japanese player is paying attention.
As far as dealing with Japanese in Siberia…my strategy is the same as in Anniversary: slowly build mobile forces (tanks, planes) with Russia in the early turns for use in counterattacking Germany. Preserve these units at (almost) all costs. When the Allies arrive in Europe, their first objective is to blunt Barbarossa, allowing the Russians to go nearly all-out against the Japanese with their mobile forces + eastern infantry.