They don't seem to be pulling it off now. Their capabilites will only degrade once the sanctions bite fully.
I'd like Ukraine to win, but if you listen to the former head of Mi6 and US military experts, they don't give Ukraine much chance of actually winning, as they're severely outgunned and outnumbered vs Russia, despite aid from the west. Despite taking heavy losses, Russia's use of tried and tested WW2 style of encircle the big cities and slowly pummle them into submission with artilerie, and endless cannon fodder plus ruthless trained killers form Chechnya, seems to be working for them slowly but surely.
And the sanctions are mostly affecting the average Russian who can't buy iPhones, Ikea furniture anymore or use Paypal, but it has no effect on Putin's war machine or on his grasp of political power in Russia. Banned western goods will still come into Russia through Serbia and Kazakhstan anyway, as they're not sanctioned countries and have liberal trade policies with Russia.
At this point, they are losing equipment much faster than they can produce new one, no one can churn out a dozen new military helicopters in 2 weeks. This alone will be a major source of attrition. Meanwhile, the Ukrainians are getting very efficient weapons from the West and they are being replenished constantly. If Russia could have prevented the inflow, they would have done so.
Sieges of big cities are a messy, protracted affair that may take years (see: Sarajevo, Leningrad). They consume enormous resources - e.g. Leningrad was besieged by 750 000 Germans. Russia isn't capable of supplying a much smaller task force right now.
What makes you think that they could mobilize "endless cannon fodder" (e.g. five million men or so), equip them with something more efficient than AK-47s, throw them into the theater and supply them with fuel, ammo and food if they are struggling to supply a mere 180 000?
Some people say that Russians are holding back and preparing winning push.
Some people say the Russians will just try to hold what they gained so far (which is few minor cities, despite maps colored red) and try to achieve their objectives in negotiations.
Hard to say what will happen.
If you analyze the actual situation, they're stalling there for good reason, nor because they can't go further. They're setting up permanent bases there so they can resupply easier, setup a field hospital and a service center for vehicle repairs. It will make next offensive much more efective, in theory.
Both of those things can be true at the same time.
The Marshall plan types work when there is guaranteed political and economical stability in the countries where there are implemented, and it worked in destroyed countries like West Germany and Japan because those countries had guaranteed stability and protection from internal and external threats post-WW2 by the US military bases hosted in those countries which made Marshall plan investments very safe bets that they would be paid off.
Not NATO, nor the US military is willing to protect Ukraine and go to war with Russia to guarantee Ukraine's political and economical stability, and as long as Russia can steamroll Ukraine whenever they feel like it, no bank or country will make any significant investments in Ukraine.