They have low debt but little ability to borrow. Once they run out of reserves they won't be able to sustain these levels of funding.

Why would they need to borrow when they can continue to sell commodities (which the EU and US continue to buy, btw)? Besides, there's no indication that they are running out of reserves. Wasn't one of Putins first moves to get off of foreign debt?

Those commodities won't even come close to covering their spending spree. And their reserves are running dry very quickly. Likely by the end of this year.

Any source for their reserve run-down?

They’re going to have a hard time exporting oil and gas if Ukraine keeps attacking their refineries. Repairing those refineries requires equipment that Russia has a hard time getting thanks to sanctions.