G5 is possible, but not with a G1 attack.
Japan can occupie russian terretories behind china in G4 and take 4 IPCs from the far east, without leaving too much planes in Japan/Mandschuria.
Sea Lion is not an option. Russia grows too big.
You are right! A combined attack on Russia is my favourite solution, too.
G5 or G6 for capturing Moskau is realistic.
We call this tactic “Panzerwalze”. 😄