Heroes 5.5 WarGame Edition - Tactical Edition.

I'm currently considering this option:
1. Dwelling: Income = BaseIncome; Growth = BaseGrowth * (1 + FortressBonus)
2. Upgrade: Income = BaseIncome * (1 + FortressBonus)
3. FortressBonus:
Fort = +100% for tiers 1-2
Citadel = +100% for tiers 1-4
Castle = +100% for tiers 1-6
Is this a simple enough scheme to understand?

It's simple enough, but personally, I think it should be 50%! A 100% increase is too much. (300% for tiers 1-2, 200% for tiers 3-4, 100% for tiers 5-6, compared to 100% for tiers 1-7 in the standard version.)

...a complex option. Is it worth it?

It's not worth it! It's better to make the simpler option more complex.
It should be designed so that almost every turn, there's a choice to be made: 1. build a new base, 2. upgrade previous tiers, or 3. upgrade the fortress!!!
P.S. I'm currently writing an example of the scheme that I like, and it will provide a more detailed "agony of choice." Once I'm done, it will be clearly visible.