It is difficult to interpret Russian strategy. What the Russian army does is gets the high ground wherever it can, plants land lines, waits for the Ukrainians to walk on land mines, advance 200 meters at a time. I don't know that the world has ever seen a slow war of attrition like this, though it does get compared to WW1 and The American Civil War.
What this task does "accomplish" is it drains Ukrainian society of able bodied men aged 16-65. This does "demilitarize" Ukraine but at a great cost. And Russia has itself lost 100,000 men.
And we know that the Russian public is dissatisfied with Putin's ultra cautious approach.
My best guess is that the Russian army is keeping most of its troops in reserve because:
- They expect an eventual NATO intervention.
- They think a slow grind is diplomatically and economically advantageous.
- Slow-boiling NATO reduces the odds they use tactical nuclear weapons.
Perhaps, but circling back to the American civil war analogy, the Confederacy was gradually losing until it rapidly collapsed. But the Confederacy wasn't being propped up by 40 countries.