It's not that I dislike Tactics. I consider it the most useless skill. I only take it in one case: when my secondary hero is guarding a Castle and wins, and gains skills there...

No, then it shouldn't be Scouting, but some kind of Navigation, so that the secondary hero can quickly explore the map by sea. Or even better, Logistics. In general, it's good for a secondary hero to have a specialty like +350 gold. You buy one, and it practically pays for itself in a week, and then it starts generating profit. 350 gold isn't much, but it's still money. And what if you have a couple of them? That's even better. Or Scouting is also good for a secondary hero.