I agree that tech stealing is powerful. As is city state election rigging. That's why I think espionage works well. It has powerful effects that are strategically significant. Each espionage decision really matters. That is precisely why I wouldn't want to create lots more 'things to do', because as you say then each thing would have to be toned down.
Can we agree on "less consequential than tech stealing or election rigging as currently implemented"? I think we agree that if you had many more things to do, each thing would have to be less significant, for balance reasons.
The flavor that I enjoy from espionage is the imperalism/cold-war feel of messing with city states. I really enjoy replicating the Great Game, and the kind of meddling that occurred in so much of Latin America, Africa and SE Asia through the cold war.
This is the most persuasive argument against many new options. Especially with the CS effects and the fun that can be had in diplomacy.
A possible compromise is to give espionage more frequent or possible small returns than a few big ones but within the same total yields, or have the big returns as options but with higher risk (as coups are versus just election rigging).