Russia is not a banana republic where you could possibly find the direct link between the export revenues and military expenses. This is not how Russian economy works, the picture is much more complicated.

First of all, the revenue from oil and gas sales is collected in foreign currency. Russian budget receives the money in taxes paid by exporters, but these money must be converted first to rouble. The conversion rate can be regulated by the central bank through purchases and sales of the currency and they of course can print money when necessary. This means that this currency actually never reaches the recipients of the budget money and exchange rate can be adjusted to serve the needs of the economy.

Second, Russia produces nearly all of its military equipment and pays for it in rouble. There's almost no mercenaries in this conflict, it is mostly regular army, which also receives their salaries in rouble. They have very low exposure to external economic shocks and if necessary can switch to the war economy mode at the cost of the rest of Russian population.

Now, what happens if Russia stops exporting oil and gas to Europe? Just in the last year Russia collected 50% more money than it planned in the annual budget. The extra money are usually deposited in the Reserve Fund and National Wealth Fund and re-invested, but they can be used in crisis times. Basically, just in the last year Russia collected enough money to survive 6 months of oil and gas trade embargo (and this is 150% of its military budget). Let's say, some of these money are frozen on European and American bank accounts and cannot actually be used except for servicing the debt. Still plenty of these savings are still accessible. Besides that, Russian central bank can still print money to fund the war. This may result in even higher inflation, but the economic bloc of Russian government is extremely competent and can mitigate the impact on the population.

Ordinary Russian people will inevitably notice the economic downturn, but their mentality is the mentality of survival and mobilization, plus the state propaganda can justify the means very convincingly and repressive apparatus can silence those who disagree. We are talking about many years of attrition until the economic situation becomes untolerable. No sanctions in the world will force Russian people to overthrow the government in the next year or two, just like they did not work for North Korea, Venezuela or Iran.