Wind of Change

So why not balance it right away so that the difference is only 1-2 days?

This has already been suggested and argued above.
After that, you write that the option allowing a difference of 6 days is excellent. What exactly do you like more about it than the option that allows a maximum difference of 1 day for each spell?

Calculate the probability that the difference will be 6 days.
Your option, where there are two 0% chances, is much worse. I really don't like the option where a player has to waste resources with a 0% chance.
Yes, it solves one "problem" by creating something like a "mirror GM," but then other problems related to "mirroring" will arise.

Maybe just reduce control (increase movement costs for casting spells, for example)

Finding a way to reduce control is very difficult. Simply increasing the number of movements or mana cost is not a solution; it will immediately lead to players focusing on logistics, stables, intelligence, knowledge hats, etc.

And as the CIS showed, we are observing heroes being rebuilt for GM.

Well, what can you do if players know the optimal strategy and stick to it?)