I don’t think contributing to CoGC should be on the premium list. It shoulders all the costs - normaly borne by all players on the planet - on a select few.
However, limiting the benefits to paying players sounds exactly like the type of P2W thing you guys want to avoid.
Tying the upkeep costs to the number of premium players on planet doesn’t change the issue, only reduces the costs.
Restricting construction only greatly reduces the impact of this being a premium feature.
Restricting construction and voting seems the best option. Leads to a situation where only the patrician elite votes and the rest must comply, but even long-established players are probably going to have a lot of basic industries on the main hub planets, so are likely to vote in a manner that benefits all. (though some people may feel like ‘no taxation without representation’, and remember what happened last time someone said that…)
Or maybe someone can think of a completely different solution.