They do have capacity to fully cover their entire military needs.
For consumer goods, they are trading with China, India and other countries in a mix of currencies, that includes rouble.
The freezing of their USD and EUR assets was a huge mistake; it demonstrated to the world that such assets are not safe.
Cars and motors are dual use goods (so the collapse of Russian domestic manufacturing of them is militarily relevant), but even setting those aside and looking purely at single-use military goods, there's persuasive evidence that the Russian military is increasingly reliant on Iranian drones and equipment (https://www.al-monitor.com/originals/2022/10/russias-use-ira...). Vast numbers of Russian equipment have been confirmed destroyed or captured by observers such as Oryx, and those are just the visually confirmed losses you can check the evidence for yourself (https://www.oryxspioenkop.com/2022/02/attack-on-europe-docum...). No country would be able to replace those losses without switching to a full war economy, which Russia has not done (and which would only be a necessary but not sufficient condition for replenishing these kinds of losses).
This is where your argument breaks down. Russia imports vast amounts of manufactured goods that it doesn't have the capacity to make itself, including basic military and basic consumer goods, and for which it needs something that their trade partners might conceivably want. Which ain't roubles, for the most part.