I kind of suspect that the equipment they had was fine where it was maintained but that most of what was paid for never arrived at front line units or was stripped and sold off after installation.
It is the pervasive and top to bottom corruption that is the root problem and the best hope to counteract it is a fairly transparent society. Which is not compatible with oligarchical rule.
It is more than that.
Russia's military is poorly trained/educated. Their economy is barely bigger than Canada's, and they are maintaining a huge number of tanks and other weapons; they simply cannot afford to modernize them, keep that many in service, and maintain them.
It relies on conscription, which you can avoid by getting a degree; this means it relies on the poorest who cannot avoid service.
It has weak off-rail logistics. Their units have something like 1/3 of the off-rail (trucks basically) logistics capabilities of "western" military units.
And then, we have the plausible story that the Tank is dead as a weapon. Modern anti-tank weaponry (both drones and man-portable) are able to kill tanks easier than tanks can kill them for 10x to 100x less cost. That really isn't a price ratio you want to be fighting against.
I mean, battleships died in WW2 with the aircraft carrier for the same reason (air planes are many many times cheaper than battleships, and can sink them). We still have ships with guns, but they aren't the battleships of WW2 and before, queens of the sea which engaged in a technological arms race for larger caliber barrels and thicker armor and faster engines. Instead they are secondary escort craft.
It looks like tanks are now ineffectively armored most-terrain light artillery and poor infantry transport mechanized units.
Not sure what to replace it with. Maybe massively reduce armor (keep enough to beat small arms; force the enemy to have anti-person and anti-mech weapons), add in drone escorts? (Refueling and controlling drones can give you around-corner kill capabilities) Stick anti-tank weaponry on some of the drones and you can beat a tank before it sees you.
Dunno.