Frankly, the best fix is twofold:
1. add the ability to refuse gifts. That eliminates the "dud specialization" exploit.
2. lock out the other specializations, even for trading. Once you specialize, you should NEVER be able to get another one of them, whether via by research yourself, trading, gift, or Galactic Event. Specializations should absolutely be single-choice only, no-going-back forks in the road.
Different tech branches for each specialization is too complex, and really too difficult to bother implementing. There are over two dozen specialization points in a typical tech tree, so unique paths for each are simply unmanageable.
And, yeah, we need to break out the Extreme Colonization planet types as individual techs, not as specializations. This has been complained about for well over a year now.