Ok, I get your basic principle, but I’m not sure the numbers add up that way.
the two tanks in western are less available, but no tank or artillery was taken from southern europe, so that’s one in its favor.
Other than that, german needs to be equally cagey here but i think they’re ok. If they take karelia with just norwegian troops, and russia musters the exact army you describe (all hit ukraine for maximum defense), germany ought to have anticipated this danger, left one guy in ukraine, and so have remaining and in range of ukraine:
13 infantry, 2 artillery, 6 tanks, 6 fighters, 2 bombers, plus any tanks they built on turn 1 (this is an important aspect players often ignore when they build all infantry armies, the importance of viable threats). So assume 2 (or 3) tanks added to that to make it 8 tanks.
this has an overwhelming chance of victory, with a likelihood of clearing with most of the air force intact and 6-7 tanks alive (according to frood, and that’s without any unnecessary loss of airpower – which is always an option if you want to double down). You’ve also captured an aa gun. Russia can retake, but with appropriate difficulty.
(Since the german is basically changing direction, the battleship and transport carrying an extra infantry, western europe tank, and bombard ability can also join in, making this a much bigger rout with 9 or 10 surviving german ground forces, a force that may not be worth attack-able for the russian)
So i agree in this example UK wasn’t forced to make a really hard choice, but russia i think was. Anytime germany can have a decisive battle reducing russia to (ultimately) just the few units they build new each turn I think it’s a good situation for the axis. Germany has the greater budget, and japan can have a bigger impact by rushing in even with limited numbers of troops supported by air, since russia has less flexibility to counter. Russia is the power that the allies can least afford to have hanging by a thread, IMO.
Anyway, all of this is to say I think russia can pretty much always force a big fight if they want to, and this strat doesn’t leave germany particularly weak since it doesn’t divert much cash.