“People greatly overestimate Russia’s ability to project power. Russia has nowhere near the forces in the region required to oppose Turkey in Northern Syria, and no way to get them there quickly. Russia would also not be willing to accept the losses they would take.”
Turkey is also a NATO member and would call on the alliance to defend it, Russia doesn’t want that.
BUT Erdogan, being a madman, is also headed towards war with Greece (a fellow NATO member) over Cyprus and if that happens Russia can come to the defense of a fellow Orthodox country without NATO being able to do anything and Assad will kick Turkey out of Syria.
Greece has a very large military and that will alter the scenario significantly.
NATO is not going to step to the defense Turkey in Syria. Russia would have to go into Turkey to trigger NATO. They are NOT going to do that. Even without NATO, It would be a disaster for Russia to invade Turkey. The losses and military embarrassment would destroy Putin.
Even in the remote chance of a war between Turkey and Greece, there’s no chance Putin would step into it.