Heroes 5.5 WarGame Edition - Tactical Edition.

but the ability to gain momentum by actively capturing outposts and, after a couple of weeks, to surpass lower-level stacks in strength. Well, it's all my very subjective view, of course.

Dragons should not arrive in packs under any circumstances; +2 dragons per week is already too much. That is, this is one of the conditions, and its fulfillment is strictly mandatory. For the same reason, it is not suitable to increase the growth rates for upgrades (as suggested in Skype).

In fact, for Tier 7, we have:
1) building a base = 100% arrival (+1 dragon) and 100% growth (+1 dragon per week)
2) building an upgrade = +100% arrival (+1 dragon) and the ability to change the upgrade
3) there is no dependence on forts

It is unlikely that something better can be devised for Tier 7, but for other tiers, this is already a question.
In principle, without the involvement of forts, it would be logical to use the same approach for all other tiers (according to points 1 and 2). That is, the question boils down to how exactly forts should affect the arrival and growth, and for which tiers.

Added 20 minutes ago
So far, the following things are obvious:
1) Forts must definitely affect the growth rate; otherwise, the decline of lower tiers in the future will be enormous (we are using the standard development model, in which lower tiers are weak and cheap, and higher tiers are expensive and powerful)
2) Forts should strengthen lower tiers to a greater extent than higher tiers (for the same reason)

Roughly speaking, a lower tier is a cheap option, quick and accessible, but it can be upgraded with expensive forts to the level of more serious and expensive tiers.

Added 11 minutes ago
Bonus size. There are two options: +50% growth and +100% growth for building each fort.

Area of influence of each fort. So far, the following options have been listed: single-step - all for tiers 1-3, and three-step - for tiers 1-2, 1-4, and 1-6.

Should forts affect the one-time arrival. There are 2 sub-questions here: should they affect the arrival from basic dwellings, and should they affect the arrival from upgrades?

If forts do not affect the arrival in any way, then building fortifications, as in the standard game, will be beneficial only on the last day of the week (since the bonus is only delayed).
If forts affect the arrival from upgrades, then building upgrades will have not only an immediate but also a delayed effect (in the future, you will receive an additional arrival when building the corresponding fort).
If forts affect the arrival from basic dwellings, then the difference between lower and higher tiers will be blurred in terms of obtaining a powerful bonus here and now. The same can be said about upgrades.

Finally, it is desirable that the final version be as simple as possible.