And one more thing. Magic Mirror – is it worth spending a skill point on it?

The Magic Mirror, combined with Chaos Magic, provides a very useful ability: reducing damage from all attacking spells by 20% (and from this skill, in turn, you can get fire damage to all attacks of your units – not bad for traditionally weak-in-attack mages).

Added 3 minutes later

The problem is that you can't build guild levels 4 and 5 at the same time, but you will have to choose a skill. You probably need to leave room for 1 skill, just in case. What if Berserk or Meteor Shower turns out to be available in the guild?

Personally, I take both Dark Magic and Chaos Magic. Dark Magic provides the ability for mass "curses," of which slowing is especially good. Chaos Magic gives such a useful ability as Lord of Cold (or Lord of Storms, whichever you prefer) – the ability to deprive a particularly dangerous enemy of their turn.