I think the gold issue could be improved with increased unit maintenance. both as a base
(instead of 0-1-2. 2-3-4, 4-5-6 it could be 0-1-2, 2-4-6, 6-9-12 (base maint modifiers would have to be per age))
And as a % bonus
Military units > 5 tiles from a friendly settlement: +50%
Military units in enemy territory or adjacent to enemy unit: +50%
+100% if it get both of the above categories
enemy meaning at war
So modern units could cost 30+ gold per turn in an active war.
(instead of 0-1-2. 2-3-4, 4-5-6 it could be 0-1-2, 2-4-6, 6-9-12 (base maint modifiers would have to be per age))
And as a % bonus
Military units > 5 tiles from a friendly settlement: +50%
Military units in enemy territory or adjacent to enemy unit: +50%
+100% if it get both of the above categories
enemy meaning at war
So modern units could cost 30+ gold per turn in an active war.