There's nothing about unprovoked attack on NATO that would help him maintain power. The current war is not popular in Russia - it's tolerated for as long as it doesn't influence the life of an average middle class Russian. Putin can't even do a second round of mandatory draft due to how unpopular the first one was. He simply doesn't have the manpower to attack NATO while keeping the little ground he gained in Ukraine. He doesn't want out of there if it means losing the currently occupied territories. He wants to act strong for as long as he can and then negotiate a peace deal.