My favorite idea was mine own for draconian tech-trading limitations:
If the tech isn't in your tech tree, you can't have it. End of story.
Maybe a little heavy-handed for some people's tastes, but frankly it eliminates 100% of the problems. The thing is, unique tech trading isn't the ONLY problem. Trading mundane technologies can utterly break the game.
For example - Torians don't get research labs, and instead rely on +% Research to advance. To that end, Discovery Spheres are NOT a unique tech, as nearly every race gets them. However, Torians getting Discovery Spheres, on TOP of their +% research bonus, go from the worst teching race in the game to the BEST. This is fundamentally broken. Being unable to get Discovery Spheres, as the technology isn't in their tech tree, fixes this hole.
Likewise, Drengin will be unable to use anything except their own racial method of industry, as it keeps with the lore as well. If using slaves has worked for the Drengin for thousands of years, and it's clearly superior to using factories (lore-wise), why would they suddenly start building Industrial Sectors and get rid of their slaves? Yeah, I don't think so either.
How about Yor with food? Again, same thing. Trading a non-unique tech to a race that was never meant to have it.
Frankly I don't see how anyone could shoot that idea down. It might sound like too much, but remember that for almost every race, around 95% of their technologies will be able to be traded! Most tech trees share all the same technologies, especially everything in regards to starships. In addition, it supports more flexible dynamics in racial relations. The Drengin and the Korath will make natural trading buddies, helping each other along, as they share all the same techs. The Drath and Altarians will grudgingly be able to share some discoveries, and the Yor and Thalans, who will be the worst-off, are loner races anyway, so it's justified.
And then, races like the Torians and Drengin will find it difficult to trade a lot between them both due to dissimilar tech trees.
Honestly now.