I explained why it isn't their source of income. They have their own currency with a floating exchange rate. Therefore what you think is their 'source of income' isn't. The FX system means that a Russian exporters 'source of income' (that which they use to pay their staff and suppliers) is the expenditure of the Russian importer, or the generosity of the Russian government sector - primarily the Bank of Russia. [0]
In local currency terms it is the Bank of Russia buying the gas, not the EU. That maintains the rouble monetary circulation in Russia, countering the drain to Rouble net savings and taxation.
[0]: https://new-wayland.com/blog/how-russian-gas-is-paid-for/
"Size" of economy is not a real metrics.
You can have a huge-ass economy based on providing high-margin services, but it would be worthless if it is not sustainable.
On the other hand, take Russia that does not really do much of global market high-margin services economy, but has sustainable enough industries.
As for problems getting anything, they cant even source potatoes for something as basic as fries.