only 5 slots for secondary skills (even if they are more extensive, but still... I think 6-7 would be just right)

Considering that Heroes is still a game with an emphasis on online battles, or at least a game with a real opponent... it would be interesting to see how these 7 slots would be fully upgraded?))
Huge maps are good for drinking beer and relaxing, when you turn off your brain and just have fun at a leisurely pace of development.

reduction of magic

in what way? if you think a little, you can see that many spells have simply found their reflection in perks or creature abilities.