This picture is often quoted and posted: Things go well, then there's war, afterwards things return to normal, at 0.4 GDP ratio to the USA.
Without the losses of WW1 and the revolution, the civil war, the collectivization and dekulakization think about how much scientific and economic loss Russia would have avoided. The wars damaged Russia far more than they did Germany, to return to the threads theme... i'm of the opinion that OTL is the best case for Germany - the geopolitical competition is gone, Germany is a near flawless democracy, it's rich, the internal political conflicts have ended and Germany is undeniably Europes top dog with secure borders and France tightly bound to it.
a: The disadvantage of being a multiethnic empire is imo overrated, it's the most normal thing outside of Europe and inside of Europe catastrophic war is needed to end it.
b: Usually around 5 % of GDP was spent on military matters in Europe, significantly more during war times. Russia too has its secret services, and they're quite good at what they do - every Bolshevik worth his money ended up in one of their gulags at one point or another - Stalin himself got arrested 7 times and exiled 6 times.
b2: On geopolitics there's most likely another war with Japan coming but this time the railway is available and makes things a lot easier.
c: Who knows, the more citizens there are the less of an impact oil exports have because less oil is exported, instead it's used internally. Without Lenin they're sure as hell not shooting the investing and innovating middle class.
d: It's still no comparison, in my own estimate based on pre war birth/death/emigration rates and a gradual decrease of the birth rate (a bit smaller due to not having the wars) and a gradual increase of the death rate they never hit 130 million even with colonial migration to Germany (I've done some excel scenarios for a few countries out of boredom, using publicly available demographic data, i dont claim it's perfect but the German population increase had sunk to 0.7 % before the war, even continuing that gives you only 80 million people by 1940).
e: Probably not, large, hugely expensive government directed programs like the Manhattan project were only possible during wartime and the resulting competition, it was WW1 which opened up the governments to higher taxation and spending, before that taxation was usually around 10 % overall tax for the big states with 1/3 to 1/2 going to the military.
f: Why would it be any more rebellious than for the Soviets? Or why would rebellions have any more impact on it? Especially if nothing's actually broken and money keeps flowing into the security services and forces. The Soviets did just fine and their situation was a whole lot harder than for the empire.
No, OTL certainly isn't the best case for Germany. The losses of both World Wars were tremendous, you overestimate the German position in Europe and you assume that Germany wouldn't be a democracy without both World Wars. I think Germany would most likely be a democracy, so I don't see how they would be worse off ITTL.
a) Not really. Being a multiethnic
empire has tangible and very real drawbacks. Especially if your rebellious populations are not intermixed with your core population and instead have their own outlying territories far away from your centers of power.
I mean if you assume that the Ukrainians, Poles, the Central Asian population groups, the Finnish people and the Baltic population groups wouldn't be unhappy and unruly we don't need to have this discussion but I really doubt that.
For me this unrest has to be dealt with in some ways:
1) "The Saudi way": Buy them off. A massive net of social benefits and tax gifts. It is expensive and seriously impacts the budget. Sure, if the oil price is high you might be able to afford it. With the crazy population growth you assume however the Russian Empire would imho quickly run into fiscal problems.
2) "The Soviet way": The Soviets (especially under Stalin) often used genocidal ways or at least authoritarian suppression too keep unruly populations in line. If the Tsarist Empire chooses this way we might see an equivalent to the Holdomor or other nasty stuff.
3) "Federalism, Cookies and Happiness": Yeah no, the Tsar is way to autocratic for that.
b) Funny. The Okhrana is generally seen as mediocre in my experience. It wasn't as ruthless as the Cheka and it couldn't rely on the massive network of informants that the Gestapo had. I really don't think that Russia could cut it with some mediocre investments into a mediocre secret police. If you want Tsarist Russia to survive into the 60s you need real money invested into oppression.
The byzantine network of militias, special police units and militarized secret service units of the Russian Federation is something we should look at in terms of cost and manpower drain.
b2) Funny. And what about A-H, with enough time they might get their shit together too. I mean you assume that multiethnic Empires can work. This would lengthen the front considerably. And a war with Japan might be very costly. you can't just brush that away with a half sentence about railroads. 1905 the Tsar nearly lost his throne.
c) Oh come on. Sure, there was a middle class but even Tsarist Russia was a bureaucratic oligarchy in the making. Germany had a culture of liberal capitalism, Russia had a suffocating bureaucracy and several well connected oligarchs.
d) I think Boonz already made this point. And lets not forget Austria and the rest of Cisleithania. Either A-H collapses and some (if not most) of these countries join the German Empire, or Russia is faced with two Great Powers.
e) You can be damn sure that taxation eventually would have reached similar levels to OTL even without
world wars. And Germany and Russia both might be very well involved in other smaller wars.
And please, the Manhattan project is just not the right comparison. The costs could have been drastically lowered with a slower and more methodical approach, I have read that with a less rushed approach costs might have been 1/3 of what they were.
Also the Großer Generalstab surely would have quickly realized the immense strategic and tactical value of a nuclear arsenal. If, like you say, this immensely populated Russia you postulate starts sabre rattling, you can be damn sure that the Germans would invest into a comprehensive nuclear program. And if all the Scientists stay in Germany, that fled the Nazis IOTL, Germany will be at the cutting edge of nuclear weapons development.
And take the pre-WW1 arms race into account. If two or three other arms races are happening between 1914 and 1960 nuclear weapons and mid range rockets are bound to be a thing. Especially fora Prussian military paranoid about the Russian steamroller.
f) Because the Empire neither would have a modern and attractive ideology, nor would it have a victory against genocidal maniacs under its belt. And I doubt that the Empire would be as effective in suppressing dissent, as a totalitarian (later highly authoritarian) socialist country.