I think this might be a great addition to the game, no longer can you supply your entire armies from one iron/copper/coal mine, especially coal and oil which are energy resources should be limited. I think cities too should be made so, that a larger city consumes more coal or oil for its powerplant than a smaller city and yes, technology should definetely influence how much resources you get from a mine, say steam engine introduces steam driven water pumps to mines allowing better mining techniques and other techniques, like explosives or gunpowder should influence mine yields, after all modern mines yield many times more resources than ancient mines, correct?
I think though, that 5 units per resource might be a bit low, but that can always be modded later I guess, I´d say something like a 100 units per resource or so.
Historically the iron ore mines of northern Sweden supplied the iron ore for the entire German army during world war 2, that´s more than a 100 divisions easily from one source of iron, thousands of tanks and millions of rifles and guns, I´d say that´s MORE than 5 units
I think Rise of Mankind mod has it good with multiple "levels" of resources, you have RAW MATERIALS like IRON, COPPER AND COAL from which you can make resources of the "next level of production" like STEEL (Iron and COAL), no other resources were yet in the mod but there could be several, like LEATHER (COW or PIG), BRONZE (copper and tin) etc. from these goods you can make PRODUCTS like SWORDS (Iron or bronze or steel), Leather armour (leather) or Iron armour (Iron or steel or bronze, better metal would give better combat strength) or MACHINE PARTS or ALCOHOL (with wheat or rice or apples etc.) the list goes on... ...and from different products you can make REFINED GOODS like CARS (engines, steel, machine parts), Cell phones (plastic and electronics), computers (plastic and microchips etc.) etc. etc. and all products could be required by units or buildings or by the people.
I think it should be so that in modern times people EXPECT more, they need more resources to be happy, not just food anymore, but cell phones, computers, luxury goods etc. Instead of one happy face resource, a population point would need two or three happy face resources to be happy or just content.
Modern troops too would need more resources, like AMMUNITION (in RoM) and BULLET PROOF VESTS or MODERN ARMOUR and ASSAULT RIFLES or just WEAPONS or GUNS
Also RECYCLING could extend the amount of resources available, for example the Germans probably melted their wrecked and old tanks for steel and made them into new ones, so perhaps the resource system should be such that every turn the mine would produce enough iron for 10 units, you could build 10 units per turn based on iron (or 100, depending on where the resource level is set), the existing units would use recycled iron for maintenance, so they would not need any new iron, I mean does a legion need new swords EVERY TURN or why could they not just melt the old swords down and make new swords out of them, they already have the iron and recycled steel is as good as new?