Putin has barely used his armed forces- his military power is vast and, if he unleashes it, it could be catastrophic.
This is not correct. To conquer and hold cities, you need to win a ground war. That requires troops on the ground, not the kind of bomb the place to rubble and fuck off home approach of the Russian forces in Syria. So you need lots and lots of well-trained, well-equipped troops.
The Russian armed forces have around 900K active personnel. Around 190K of them - more than 20% of the total armed forces - were involved in the initial invasion. Many of them were the highest quality end of the armed forces, e.g. paratroop regiments, though there also seem to have been a lot of totally unsuitable conscript troops. Everyone - elite forces and conscripts - took an absolute hammering in the first phase of the war. The Russians have been redeploying from places like the Eastern Military District and Russian-occupied South Ossetia in Georgia, but still seem to be short of troops - the Pentagon assesses that they're still not up to their anticipated strength in Donbas, despite several weeks of the much-touted 'phase 2' of the war.
Many of the approx. 700K (probably less now) active personnel not already committed are not frontline troops. We've already seen the consequences of putting non-frontline troops into frontline positions (goes about as well as you'd expect).
Even in a war, Russia can't just try to put all its troops into Ukraine. It has ongoing interests elsewhere that need defending and it needs to keep a lot of men and equipment for the defence of Russia itself. Putin tried and failed to push Belarus and other Collective Security Treaty Organisation states into committing troops. Sooner rather than later Russia is going to be at the outer limit of its capacity to reinforce troops in Ukraine from active personnel elsewhere.
So then the next stage is to move to reserves, of which there are many, and beyond that to a call-up of other fighting-age men. But it takes time to get reserves up to speed and much longer (months) to make conscripts ready to go into battle.
Meanwhile, the remaining professional forces in Ukraine are being ground further down and more Russian equipment is being either destroyed or captured. A few weeks ago, the Pentagon estimated that the Ukrainians had more tanks than they began the war with, because they'd taken so many off the Russians.
Russia's ability to quickly replace men and equipment is degrading. Ukraine is not getting troops from NATO but they're on the receiving end of staggering levels of military aid from the US. Russia simply has no capacity to compete with that.