Wouldn’t be use of biological WMDs by Ukraine within Russia fully justify the use of nuclear WMDs by Russia on Ukraine? I think it would be a flash of brilliance by Ukraine, similar to bombing the Kerch bridge or sinking the Moskva, but resulting in a counterattack that would (simply put) end Ukraine.
With that said, unless something drastically improves, I can see Ukraine having to pull a Hail Mary move to try and engage real help. I think an attack on the nuclear power plant or in a NATO country (by Russia, Ukraine, or some unnamed third actor) would do the job. Only the full involvement on NATO will stop Putin. Sending MBTs and Vipers will not do it.
That wouldn't even do it. It would be another Vietnam at best.
Who wants a hot war with Russia and, potentially, WW III or nuclear war? It's insane anyone would even want to consider any of that.