It’s hard enough having a romantic relationship between two people

well the other realtionships don't have to be romantic. They can be sexual, they can be about platonic affection, they can be about kinks your other partners don't want to engage in. Working relationships like that often have very clearly seperated roles for different partners, and i don't know any that have worked out long-term where both nesting partners slept with the same people. That idea that polycules are a bunch of horny orgy goers constantly fucking everybody in the polycule is one that's pretty far removed from how polyamory usually plays out when it plays out successfully.

Although i have to admit that i don't see any of that working within the norms and expectations and cultural baggage our society has for straight couples. This mostly tends to work for queer people who didn't have normative ideas of their preferred kind of romance shoved down their throat their entire lifes and had to find out their own way to make love and sex and romance work by just trying things out and coming to grips with that they want and feel.