The answer is simple, yet so obvious.

Make units cost more in maintinence.

Civ 5 works pretty well in the early game. You're ruthlessly economizing everything in the first like 100 turns as you go from Ancient to Classical and medieval.

It is the late game that is a chore. And that is because of unit numbers. Basically, by the time you get to late game, if you want to conquer a city, you have to build many many more units. In the ancient game, a smartly manuvered army of like 3 swordmen and 2 catapalts are enough to wage an effective assault on an enemy civ.

By the time modernity rolls around, you need at least 20 units built up to invade an enemy civ. This is because units cannot attack more than once unless they have a late game talent.

20 units is annoying. It's a lot more clicking. Finess goes out the window. It's just not fun.

The solution I believe is in unit maintinence. Modern units should cost more in maintinence. This makes sense. By the time modernity rolls around, I'm usually running hundreds of gold in surplus. So, I don't even think about it when I build more units.

Because modern units only take up 1 gold per turn in maintinence.

In ancient times, if I overbuilt, I'd be facing a deficit quickly. In modern times, that's not a problem.

So, in modern ages, the units should be more expensive. They should also all be able to blitz I think.

This reduces the number of units overall and that alone would solve the late game.