I think the strategic ressource improvements, specifically the durantium refinery, thulium data archive and promethion reserve need some rebalancing. Right now they are pretty strong and its hard to argue they should not be build them on nearly every planet that can sustain them. They also punish the AI which is crappy at improvement placements to maximize their +production potential. There are a few options:
1. Lower it to 2 base and keep +1/level improvement. Then these would only be worth it on larger and highly specialised planets.
2. Lower it to 4 base and lower it to +0.5/level. They would still be very strong and generally worth building, but would be more useful on larger specialised worlds.
3. Keep it at 8 base and change it to +5%/level to manufactuing/research/wealth. They would still be strong on both large and small worlds alike. Good for the AI that is weak with their improvement placements.
My second caveat with the improvments is the promethion pleasure park, which is nearly identical to stadiums and inferieur to the drengin or altarian level approval buildings. Options are:
1. Keep the +6 approval base and increase it to +2/level. This means a pleasure park dropped on a haven can keep the whole planet happy.
2. Change it to a +% approval and keep it at +1 approval per level. I would recommend +100% approval. (The first malevolent motivation ideology building gives +50% approval and also gives ideology points, so I dont see +100% at the cost of a strategic ressource to be a disadvantage).