The "limited tech trading" option sounds good in theory, but the problem with it is it will likely significantly disadvantage the more unique races out there. The Thalans for example have such a highly exclusive tech tree that it will largely leave them out in the cold. How do you balance this option?
I'm the strongest advocate of this idea, because despite your nay-saying, you need to look at the big picture.
The absolute rule for tech trading should be that if you can not research a technology yourself, meaning it is nowhere on your tech tree, than you will be unable to trade for it. Plain and simple.
You're vastly overstating the disadvantages to this. The biggest disadvantage is that game exploiters (ie: metaverse players) will be severely hampered by their inability to abuse the crap out of tech trading as much.
Secondly, nearly every race will still be able to trade most, or nearly every technology they have. Believe it or not, the Drath and the Terrans share most technology. So do the Altarians and the Arceans. Governments. Weapons. Defenses. Colonization, ship hulls, etc makes up a huge part of the tech trees, and is mostly shared by every race. In addition, this will set up natural race relations. The Drengin and the Korath share nearly every technology, so it's natural that to advance the Drengin tree prematurely, you'll need to do some trading with those whose technology you're familiar with.
Third, this, obviously, eliminates all unique tech trading. The reason a technology is UNIQUE is because the entire theory behind it, as well as the technologies its built off of, is completely incomprehensible, unable to be replicated whatsoever by another race. An Altarian technology, for example, could use their innate magic to work. Therefore the Terrans would be utterly unable to use it, even if they had every single blueprint for it. Arcean Weather Control could be the end result of a series of technologies that has taken 1,000 years to perfect, and therefore there's absolutely no way they'll give it up.
Finally, this avoids the most obvious exploits - ie: Torians with Discovery Spheres, or Arceans with HyperWarp. Or, again, Thalans getting a 'normal' start, forgoing their INTENDED technologies with 'mundane' techs. Thalans are easily playable without tech trading, to do so is a crutch for a crappy player. If you absolutely need technologies so bad, just Ctrl-R cheat for them - after all, that's what 90% of NORMAL tech trading is - exploiting the AI, abusing the game, and ultimately cheating for a significant, undeserved edge, an edge that severely needs to be trimmed.