Which magic schools should I choose for the late game? Or should I go for a specific artifact?

KillAndAction
Thanks for appreciating my "titanic effort," it's nice to hear! It just took me about 5 hours to write the last post. That's how a 5th-year student, who also works as an English teacher in an elementary school, spends his free time.)))
Regarding online play, Summoning can be very good if the battle takes place at levels 10-20. Phoenix is strong and will be a huge problem for the enemy army due to its initiative and HP. It can keep the enemy occupied for a long time. Traps are also good up to level 20, while the armies are small and the damage is still significant. And I agree about the Fire Warriors, BUT you can never predict the hero's stat growth. Sometimes one hero's magic grows faster, while another's doesn't grow at all, and all the points go into knowledge. Sometimes, as if by Murphy's Law, you level up Summoning or Chaos, but the magic stat is low. Then you take Light or Darkness, and the magic stat starts to increase.

Added 14 minutes later
Regarding the late game, I liked my Ogre build, but Summoning can be removed, as it's unnecessary. It would be better to take Logistics and Pathfinding, but not develop them further. Although, you could take Surprise Attack, but Logistics only gives 2%, so you can't rely on it.
Darkness can replace Chaos and Summoning if you have Subterfuge and Berserk. )
This way, you can destroy a larger part of the enemy army. It's great that you can use both at the same time on different enemy creatures. Blindness is also a good ability.
And the worst thing is 40% Haste + 40% Slow and Mini-Arts with 20-40% initiative... I tried it.))) I attacked the enemy 6 times, and he died without even getting a single turn.)))
In short, Light and Darkness are the best choices for the late game!
Spell Power is a great choice for online play, but only if you have time to level it up. Otherwise, you'll be without magic.