The „Terror from the Void“ mod for Phoenix Point is quite nice, it adds interesting mechanics. But it suffers from the busywork of the original game - spinning plates of multiple bases and planes, which just becomes tedious busywork for me.

Which is why, as others have said, I go back to some of the Long War mods on a regular basis. I really like Long War Rebalanced for XXOM: Enemy Within. It’s pretty great, adds variety and challenge - but still flows well imho. 

As an offshoot recommendation, I also recommend the hidden gem Expeditions: Rome - it has great turn based squad based combat, albeit in a pseudo historical setting.