alt Hacker NewsUkraine has already launched several mass strikes on Moscow.
https://www.pbs.org/newshour/world/moscow-comes-under-one-of...
https://www.atlanticcouncil.org/blogs/new-atlanticist/russia...
Even if Russia sees a particular tactic or weapons system as an existential threat it's questionable whether they have the capability to escalate further. I mean they can threaten nuclear strikes on Ukrainian population centers but would anyone believe that the threats are credible?
TBF a proxy of one of the nuclear superpowers (ie Ukraine using US arms) is quite different from a run of the mill non-nuclear country retaliating against an invasion using conventional arms manufactured at home. The former invites MAD while the latter is predictable and boring. Seeing as they are the substantially larger aggressor presumably they can pull out of this war of attrition whenever they feel like it.