The Russians did have a very modern armor column hanging out just north of the (actual) Russian border with Luhansk which could wreck a fair bit of havoc ~ at least one full armor battalion tactical group including the latest T-90M tanks ~ except they appear desperately short of infantry and supplies to properly round it out. (And Russia has finally learned - maybe - that unsupported armor is just Javelin bait).
Blowing up all those rear ammo dumps plus counter-battery capabilities appears to have given the Ukrainians near parity on artillery cover (which is a major change). Normally would expect the Russians to counter the advance with mass saturation artillery ~ but they apparently can't right now.
I still can't get over that Ukraine appears to have achieved basically parity, or maybe even the advantage, now in both artillery and air coverage over the battlefields ~ which is ridiculous given Ukraine has exactly 0 safe airbases, Russia has a half dozen completely safe regional airbases outside Ukrainian reach (although Crimea should have been outside of reach, oh well...). Russia had at a minimum 10x the artillery, with several multiples of that in reserve, and Russia still retains vastly more aircraft than the limited Ukrainian air force.