Star Ruler 2 did "strategic resources" very well, imo.
You had the occasional world with "marenium" or whatever it was called that let you use special reactors on your ships. They weren't super good by themselves, better than base reactors but maybe slightly worse than antimatter ones, but if you developed the marenium world, any ship using a marenium reactor would gain an ever larger bonus, with the caveat that if you lost the marenium resource, those ships would take a pretty big penalty.
It wasnt so critical that you had to have it, but it was a nice bonus, it saved you from having to tech to antimatter and if built up could even be better than antimatter, but an enemy taking it could severely hurt you if you focused too much on it.
It also had artifacts that, when activated, gave you massive bonuses (like, 50% more health to ship parts massive) but could be destroyed to revoke those bonuses until activated again, and activating them is EXPENSIVE.
Then again, borders in SR2 are a lot more... fluid, than in stellaris. More of a suggestion than anything, really, and SR2 is very firmly a 4x and not grand strategy. I only mean to point out that rare strategic resources can absolutely be done well, make a VERY good focal point for conflict, and still remain fairly balanced in the end. If a player is building up a marenium planet, you better believe he's getting dogpiled before it gets out of hand.