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

I haven't heard of that.
Yes, if we're talking about RTA, Haven is practically unbeatable there.

So, why isn't there a thread about RTA? Well, I'll post here then. *For those who play the RTA map* Is it true that mages are the weakest faction there now?

RTA is very different from the standard mode. Even the upgrades are different, and they are not the same as those in the standard mode.
In the latest version, Half-RTA 1.1, mages have become much stronger. In versions RTA 1.7 and Half-RTA 1.0, traditionally, in my opinion, the strongest are:
- Elves;
- Orcs;
- Necropolis;
- Humans.