Military pay (regardless of currency) and production of war materials cost money, and its a cost that can more comfortably be borne by an economy performing well. The entire idea of sanctioning exports is the theory that without excess revenue, an aggressive government would be forced to cut expenditure on an unnecessary war.
To add to this, while Russia's military expenditure dwarfs Ukraine on paper, it's mostly going on things that have no bearing on the current war. Maintaining a giant nuclear arsenal is expensive. Developing new launch vehicles for nuclear weapons is astronomically expensive.
I think what we're seeing with the frankly fairly dismal performance of the Russian military in Ukraine is what happens when you try to be the Soviet Union with half the budget. The money is all going on maintaining these expensive symbols of national prestige, like the nuclear arsenal and the navy and the giant stockpiles of ex-Soviet military hardware. The basics, the fundamental capabilities that go into fighting a modern conventional land war, just aren't there.
And for most countries that would be fine, because most countries don't see themselves as superpowers. They focus on developing the capabilities they do have rather than trying to out-spend the US and be good at everything, because they don't expect to ever fight a war alone. Russia, for some reason, has decided it is still a superpower and needs to have all the capabilities the US does on the budget of the UK, supported by an economy the size of Italy or Australia (and which is entirely reliant on energy exports to its supposed rivals in Europe).
A lot of the problems with the Russian military are too late to fix now, and I don't think the government is capable of making the kind of cuts to prestige projects that would allow for bringing the army up to the standard of other large European economies with the budget they currently have.
I don't think Western sanctions will render Russia unable to fight this war in the short term, the military hardware is already there and governments have ways of bleeding their populations of money for whatever they need. But unless the Russian government and military staff are deluded enough to be happy with the way the army has performed, they're going to need to start putting more money in if they want to be taken seriously again, which is obviously an issue if the money isn't there.