The essence of the tactics when playing as the Dark Elves.

Well, I just tried out Lethos and went to the center on day 4. And I already had Fire Warriors. I cleared the Northern Riders and High Druids on the bridge. So, who's going to stop me from getting there? Sure, mana is a bit low.

You cleared the guards, you got lucky (really!) with your Summoning skill, but what will you do in the center without mana? You can clear the bridges with Kaspar on day 2 if you're a little lucky. But he's only truly combat-ready in the center on day 4. Either way, that's two days of leeway.

I would have leveled up the Guild to level 2 and gone there on day 6 with powerful spells and a way to clear Utopia.

Let's talk about that in more detail. How are you going to take Utopia with 20-30 mana? You're not a Necromancer who found an artifact in the center, fought some Topla, and replenished his mana. Of course, if you got a Helmet at the start, found Wings (+3 knowledge) during respec, and got something else from the upgrades, then maybe. Do you know some secret word that makes such good luck happen?
Even if you're lucky with your Summoning skill, you get a Crystal, and you have mana, you'll take Utopia with 2 stacks, but after that, it's highly questionable because you'll run out of mana.

And by the way, the Necromancer's Helmet isn't that rare at the start. So, I would have 40 mana.

In a real game, I've never found it.

And I would easily fight Kaspar.

Only with full mana and powerful spells, but you don't always get to choose the time of the encounter and the necessary mana costs. And what if it's already the beginning of the second week, and the Necromancer has raised 15 additional vampires, which is not uncommon on this map? IMO, it's better to sacrifice Lethos but ban Kaspar. The Elves have other worthy heroes. Of course, the Necromancer has Zoltan.