Heroes 5.5 WarGame Edition - Tactical Edition.

Which growth system is better?

The one you have implemented now, I like it more. When a large number of high-level creatures appear at once, in my opinion, it violates the game logic (strong, expensive troops should be prepared for a long time and earned). I don't know what this would lead to in terms of game mechanics, but the opposite situation would look more logical, when high tiers work for the future, and smaller and more numerous ones can be used for rushing, getting a lot of them almost immediately after the building is built (built barracks, gave everyone crossbows - and crossbowmen went to war, it's easier than preparing cavalry or training, or even raising a griffin from a chick).
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I also want to add that from the point of view of balancing tiers, you can really connect and take into account the strategic part - external buildings. In the original (I don't know how it is with you now), there is a mechanism that each external dwelling brings +1 to the growth in the city, which significantly affects high levels more than low levels. All this, in my opinion, can be used. Let high levels have a small growth and a small (maybe even zero) increase from the upgrade, zero reaction to the fort, but the ability to accelerate when actively capturing outposts and, in a couple of weeks, to overtake low-level stacks in strength. Well, all my vision is very subjective, of course.

Well, we are no longer tying upgrades to forts in any case, that is, they are fixed, right? The question is only whether the growth will be the same as from the base, then the upgrade gives too few bonuses, because, unlike the base, it does not bring weekly growth.

Maybe you can adapt some alternative bonuses for this? If you think in terms of my remark about outposts, you can increase the growth of high-level creatures in the city from outposts only if there are building upgrades. If such a subtle thing is feasible.