I think the momentum may be lost at this point
There have never been any momentum. Russians had 2000 troops in Kharkov.
2000 of Eastern Ukraine volunteers. It had been pretty much abandoned at the time of that great offensive. Pretty much all 2000 left unharmed while about 2-3k of ukrainians soldiers were killed in that offensive.
Momentum is a creation of western propaganda which needed that to keep public support of Ukraine high.
So what? The reason
why Russia only had a few thousand sub-standard troops was because it was too stretched and made poor strategic choices. Like I said, it doesn't matter that Russia fucked up, it's still a victory to take advantage of that fuck-up. And the "momentum" wasn't fiction. Anyone could confirm it from simple fact of Ukraine liberating town after town in a quick progression. Even Russian propagandists went completely nuts over the sudden loss of Kharkiv.
As for casualties, you know as little about them as I do.