Defeating an enemy, especially a larger one requires finding their Achilles Heel and hitting it hard. Russia's supply system is great for defending Russia, but it's absolutely awful for operating outside of Russia.
The entire southern front is completely dependent on supply moving across the rail bridge they built across the Kerch Strait. Cut that line and the only way to move anything from mainland Russia to Crimea and then up to the southern front would have to be moved by ship or plane. The Black Sea fleet has a handful of landing ships they can press into the job, but they would only be able to move a fraction of the supply they are moving now.
Hitting heavily used rail bridges within Russia would also slow down supply in general, but the Russians have a large number of rail service people whose job is to repair knocked out rail bridges and other snarls on the rail system. The Kerch Strait bridge would take a lot more effort to replace than a rail bridge over a river or a road within Russia. It would probably take them a couple of months to repair it. And the Ukrainians could hit it with another missile from time to time to keep it knocked out.
The Kerch Strait bridge is a big bang for the buck target. The Ukrainians have already said they want to take it out, they just haven't had the means until now.
One reason the Ukrainians have done as well as they have is because of their ability to hit Russian logistics and supply as well as their officer corps.
Russian leadership is another Achilles Heel. Russia is a very top down army. Eliminate the leader and the soldiers will just sit there and do nothing. Nobody take initiative because Russian leadership hates that. The Russians have always feared their troops thinking for themselves because if they do, they might turn on their leadership.
Back in 1976 a Russian pilot defected to the west by flying his MiG-25 to Japan. The Japanese gave back the plane (after the US had a look at it), but the pilot got asylum in the US. I read a biography of him some years ago. He knew the Russian system was broken, but he had a hard time believing what his handlers were telling him. There were two things that convinced him they were telling the truth.
The first was when they were taking him to a safe house, they stopped at a supermarket to pick up supplies. He disappeared and the handlers had an initial panic. They found him staring at the meat case. He said he couldn't believe they had all that meat on display and neither was none of it rotten, but nobody was stealing it.
The second was they offered to show him anything he wanted to see. He wanted to see a US aircraft carrier up close. He watched deck operations on the carrier and he was convinced that the US military was vastly more professional top to bottom. On a carrier deck, the person in charge is an enlisted person. Pilots salute just before being launched and they are asking permission of the deck captain to leave the ship. An officer asking an enlisted person because the enlisted person is in charge of that task.
He watched how everybody worked as a highly skilled team to cycle planes on and off the deck and commented that he knew that would be completely impossible unless everyone on the deck, in command of the ship, and flying the planes trusted everyone else completely. He said that operating a carrier any way like that in the USSR would be impossible. Nobody trusted anyone else.
Supply and leadership are not Achilles Heels in more professional armies. The US is hyper focused on logistics and supply and always have plenty of supply units, plenty of supply on hand, and plenty of options to deliver it. Most of the US's allies may not be quite as hyper about it as the US, but they are all pretty good.
Generally democracies give all their troops a lot of permission for initiative on the battlefield. Heavy losses in leadership will be felt, but because everyone knows what they are supposed to do and are given permission to innovate, the loss of a leader will degrade the mission a bit, but everyone else will step up and do their best to accomplish the goals. Sometimes the person who replaces the fallen leader turns out to have more talent at leading than the person they replaced.
EDIT: New Perun video just up
This one on how corruption weakened the Russian military. For those who don't know, Perun has a series of videos where he does a deep dive into individual topics about the war. If you want to understand these things in depth, I recommend them.