That is true, bombing targets in a NATO country wouldn't work. On the other hand Russia could probably try to completely seal that border. It's not easy to do and will take years, but can be done.
They don’t have the troops to counter civilian resistance either. I read an article about hostile army : civilian population ratios in recent conflicts. Russia is off by an order of magnitude and Ukraine has a history almost as bad as Afghanistan. Russia really thought they would be greeted as liberators.
Even if they did, they won't now. And that only means more bad news as then they wouldn't mind a Syria in their backyard as the only thing that's worse than that, in their eyes, is NATO in their backyard.
> ...history almost as bad as Afghanistan.
I don't know what history you are alluding to, but after advent of Islam in Central Asia, Afghanistan was a dominant power in the region, with its rule firmly established in South Asia for a century or two. Afghanistan's recent "bad history" is courtesy of Soviet and American invasions.