You could make it that you need to have X% of the harvesters in the world instead.
Problems with this is:
a) Victory can be too easy from pillaging all the enemies. I'm not sure that this is good for gameplay, because the AI doesn't know how to defend its harvesters.
b) Non-harvester spice sources aren't counted. I think this is strategically non-optimal (things like Project Amal *should* count towards spice control) and confusing for the player, since they don't observe the number of harvesters they control (without counting manually), what they observe is the number of spice resources in their cities (from all sources).
I wouldn't see this as a hybrid method. It is just a victory condition with two fairly simple to understand conditions.
I'm not particularly bothered by the dual condition, as long as you can find a transparent way to display both to the player (in the victory screen).
build numerous Infiltrator units and sabotage enemy AI spice harvesters without declaring war or having a large offensive military force
The AI isn't going to be able to sabotage harvesters; the destroy improvement espionage AI only targets only-land bonus resources. We can't be unrealistic about what espionage is going to accomplish.
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I think we might be able to get away with something where pillaging harvesters helps, as long as the proportion is high enough.
I would make it:
Count up the spice resources available to each player in their capital. Call this sum Z. This INCLUDES spice from project amal, spice purifiers, etc.
You must have X% of Z AND at least Y spice in your capital.
And X will need to be *high*. Like 80-90%. So, you can accomplish the goal through going on a massive harvester pillaging war, but you have to declare war on virtually the whole planet to do so.
This means that the victory condition WILL require you to play differently. Not just normal expansion, but a concerted effort to simultaneously destroy enemy harvesters.
And Y might be say 40, scaled by map size.
The AI is never going to achieve the victory condition, but that's not really the end of the world. We aren't really designing a game where the AI has a decent chance of winning, merely where the AI has a good chance of frustrating the player from winning.