Heroes 5.5 WarGame Edition - Tactical Edition.

I was also thinking about something similar (to declare war on all sworn enemies), but then there wouldn't be any point in leveling up the racial skill itself, only for the perks.

Well, that's the point – not all of them, but only the first three levels.
That is, the racial skill works immediately when declaring war.
And, at the same time, it provides some protection against a rush. When someone like Orson arrives early with a crowd of low-level skeletons/zombies.
After that, you have a choice – either leave those three levels as they are, or declare war on higher-level units.