Like the Russians, the Iranians seem to have concluded that there was no point fighting the new Syrian army if the old one just dissolved. May as well hit the reset button and go from there. In terms of the Iranian position, I think negotiations with the Turks are paramount - both share a Kurdish enemy and increasingly it seems both may share an Israeli enemy. (With the Kurds, I see drone video of the Turkish army going to town on Kurdish forces all over Syria right now. They are defenseless against Turkish drones and require US assistance, ie, NATO vs NATO.)
With the Russians I would suspect their dipilomatic hand is vastly improved, in that they can concentrate on making moves to win the war in Ukraine without having the added complication of propping up the Syrians, which presumably required a delicate balancing act between competing interests. Now, with Assad gone, the Russians are free to wheel and deal with the Turks in Syria, whereas before with Assad's enmity towards Erdogan, they could not.