Indeed I prefer the 3.81 tribute mechanic more than the Spellbook Summon (also because that would save a spellbook slot for another power). But I still think he should remain permanent if the Goblins give him the Ring for a simple reason:
It's the fucking Ring of Power, the One Ring of Sauron XD
Smaug is not stupid, he certainly knows what is that (he even suspects Bilbo is carrying a very powerful treasure, even if he never saw him before nor he knows Bilbo actually has the Ring)
Also, speaking from a technical point of view, if you keep the Tribute mechanic active once you give him the Ring, what happens when Smaug runs out of time and disappears, and with him the Ring? It would be a waste to use a Ringhero just for a small amount of time (that's not the case of Boromir, for example, since he remains longer on the battlefield and when he runs out of time, he simply dies. And by the way, he has a different role: he's a tank, while Smaug is a mass slayer and building destroyer)
So, I think the Tribute mechanic is perfect for his character, but it should be gone when he gets the One Ring for two reasons:
1. Having to wait for his return to use a Ringhero/collecting again the Ring if you choose to make him drop when he leaves the map would make the game very frustrating and unbalancing
2. He has acquired the ultimate destruction weapon, why should he leave the battlefield, which is also full of enemies to murder?
(I also like the idea of sending tribute carts in a similar way to the Dwarven Expedition camp, but I think that can't be implemented, just because it's been already used by the Dwarves, indeed)