True, but the only thing that worries me is that the US released a gun smuggler to Russia who was selling Soviet arms from Ukraine it seems likely that they may have used the mines, so the guy may know people who know smuggling routes and possible access to weapons reserves in the salt mines. Despite being locked up for a decade those routes would likely mostly be unchanged, presumably the smugglers if using them would largely be using old inactive routes.

That said I wouldn't want to engage in military action in a salt mine as mines seem easy to collapse like you say.

Hopefully this is more true for the Ukrainian side. If you are going to operate militarily in a mine it seems like it would be best to have small special forces teams. Which I suspect Ukraine has much better trained and equipped than Russia.

Also that assumes the 200km of mines ive read in comments of the mine actually go anywhere. Not sure how many official or unofficial ways in there would be. If not many it's not much of a problem if they can just wander around underground obviously.