Hmmm,.. I think maybe you could fix some of the problems by dividing tech into "discovery", "research" and "application" parts.
"Discovery", you would get more or less automatically by interaction with another civilization. The more use a technology sees, the harder it is to keep it secret.
"Wait, the Terrans use laser technology to make 3D pictures?
"Research" is a bit harder, because even if you can buy a gizmo on the open market, the odds are it won't come with a scientific paper attached. Still, you could simply have your diplomats go to the local library, Again, easier with wide-spread older tech, and if you have a good relationship with the civilization in question.
"OK, now I know what the theory behind holograms is."
"Application" is the hardest part. It's not like you can plug your alien gizmo into the wall. You need to produce the tech with an industry base that's nowhere like what the other civilization uses.
"But how do I get it to work in liquid methane using biocircuitry?"
Basically, even if you acquire alien tech, you still have to spend research points adapting it to your specific needs, making it not an instant improvement but something you have to work for. If you trade for ten techs, you'll still spend the next forty turns or so deciphering how to produce them; more depending on how different the other civilization is from yours. (Adaptation costs could differ for different techs, making some techs more desirable to trade for.)
Application can also be an ongoing process, making your existing tech better; The new quantum computing autopilot you got may have fancier piloting skills, but the older one takes a lot less space, is cheaper - and can take a hell of a beating, too, before breaking.
This sort of obstacle makes it less profitable to trade tech - but it's still important to do so; you just have to pick and choose what techs to spend resources on. And a more diplomatic species will still get more tech discoveries - only it's done automatically, with a small chance to get one every time you trade with, conquer or otherwise interact with members of another species.