I havn't really tried being a Science Broker that much.
I like keeping the AI in the dark ages. I just got tanks and they just got Nationalism, Medicine, Sanitation and Gunpowder.
(4 AIs and those are thier new techs.) They have had to discover a lot on thier own. I've sold them a few and when I sell, I sell the same tech to everyone.
What I've done to bankrupt the AI is keep a strong monopoly on resources. I discovered Coal first. So I went and grabbed all the coal. Did the same with Rubber except there is another one and the Americans (who just discovered gunpowder) have it so they are a ways off using it.
When I go to war I will first cut off the capitol so they can;t access thier own resouces and then take the cities with the resources and other Strategic spots like chokepoints. Then they have to pay me to keep thier people happy. And then they will do anything for 20 turns of Saltpeter. But even if they have it for 20 turns, they are so finacially strapped they only make a few musketmen and then we are at a new round of negotiations.
I do have friends in the AI community. I've rescued the Americans a few times and they have been "Gracious" for about 50 turns.
I own all the Coal, Oil, and all but one rubber. The only srat. resources I don't have a monopoly on are Iron, Horses, and 2 Saltpeter. (The saltpeter invasion is next.

)
The only lux resources I don't have access to is Dyes but that is in the next city on my Indian front. And I've got a monopoly on Gems, Incense, Silks, Wine, with everything else access to. I only go to war to deny the AI something.
I'm about to Invade the Russians (who are getting close to military tradition and I don;t like the idea of fighting Cossacks). They have the other 2 Saltpeter and they have the only other Furs on the map. So I want to deny them and thier allies Furs and Saltpeter.
I've been at war for over 50 turns as a republic with police stations everywhere, Women's Suffrage and 30% lux on top of all the lux resouces (except of course Dye). I don't have a War weariness problem yet but it is noticed. Most of my war are fought against AIs who have stabbed me in the back.
So I guess I have a diffrent strategy and I don't go 100% for the Tech Broker strategy nor do I go for any one strategy 100% except perhaps Monopolization strategy.