Good points, CieZ.
Solution 4: Limit 1 heavy tank at a time per player, but remove fuel cost.
This way building T3/T4 units mid-late game is not discouraged since it will not delay the super heavies. Also prevents super late game call-in spam. The other options don't stop the late game heavy spam it only slows it down. With one heavy at a time (and for no fuel) its also still encouraged to make T3 and T4 units even after your call-ins are unlocked for more support rather than saving up for double (or even triple) heavies, which would inevitably happen in longer games.
Thats what i tought.
I think the issue is the competition for resources (fuel). If there is an option A and an option B and B gives you a better unit then you will go for B since it is more effectiv. Thats why "Solution 1" wont work imho. At the end you just increase the price and delay the heavy but you still chose it.
An other idea: Maybe even just heavy one per game. It would be mroe important to keep it alive and you you don´t save your ressources for a new heay this way. You can spend it for other units. I also like the idea to reduce the ressource income for some time if you decide to go for such a heavy.