I do not think restricting access is necessary for gameplay purposes.
I will repeat my earlier concerns:
Forcing an AIP increase at really high difficulties may be too much of a punishment for the bonus you get for a small number of nebula.
The current deepstrike system (10 threat/sec at Diff 10) means the player already has to pay a significant price to do nebula without popping planets.
I think making the AI stronger (Exowaves, AI Champions, more reserves, etc) is only useful if it replaces the already existing 20% bonus the AI gets. Really, that's more than enough of an AI bonus. It doesn't need any more. In fact, that bonus needs to be tweaked, IMO.
I think a better alternative would be require the player to do something else, the same way the Fallen Spire campaign does with the Shard retrievals. That would make it more fun, and restrict access.
Ideas for that:
Have a building on the homeworld that players would use to construct a 'Weak Wormhole Detector' or 'Drive Calibrator'. Make it cost something large (1,000,000 resources, or 250,000 * #_nebula_completed or something).
Have the Champion required to search out the nebula using a Large module that causes the wormholes to appear. Being forced to search out the wormholes would put a significant delay into the progress.
Have the AI have a building that blocks access to certain wormholes. The player would be forced to search out and destroy this building to continue (similar to the Antagonizer).