I don't mind 1 UPT that much, but it certainly isn't perfect. My preferred system would be armies - several units grouped together which move across the map as one. The number of units which could be grouped like this would be limited, and the limit could be modified by a number of things, such as tech/civic development, leader/civ abilities, or the abilities of the army's leader. Many games have systems like this, so I will not go into great detail, but some good examples would be Fallen Enchantress, Age of Wonders 3, Stellaris, and Conquest of the New World. I think this solution is a good middle ground which resolves both unit congestion and stacks of doom, while at the same time making the military aspect of the game a great deal deeper and more enjoyable. Battles could be resolved either through a tactical combat layer, or a good autoresolve function which takes things like unit synergies into account. I personally enjoy tactical combat, although it may shift the focus of the game too far away from empire management. I think Fallen Enchantress and AoW3 are good examples of somewhat complex tactical battle systems, while oldie CotNW had a simple but enjoyable one. There's also a risk of messing this up, and in my personal opinion, Endless Legend was an example of that.