So I am like 60 runs or more and the games difficulty drops quite a bit if you just invest in the mirror.
Yeah, I really feel like some of the mirror upgrades are huge game changers. The combination of "get 3 health when passing through a door", "get 3 death defies", and "start with +50 max HP" means you should almost never die in Tartarus or Asphodel anymore.
It's a bummer that you have to divert so many early game keys to unlocking the new weapons, because if you just put enough keys into the mirror to reach that "start with +50 max HP" perk, it's such an enormous boost to your early game survival.
As part of this line of thinking, I'd probably recommend that early players take any Poseidon and Chaos boons that give extra darkness and gems, because your first 10-20 runs you just need to farm a lot of upgrades so that you have enough support to actually reach the later parts of the game.
Yeah I was always taking those boons. I really approached my first 20 or so runs as farming runs, which made the later runs much easier.