Since I just wanted to experience the full story I set the difficulty at POTD without scaling and without any Magran's Fires challenge activated. That being said, Forgotten Sanctum was such a challenging yet enjoyable treat even without upscaling and any Magran's Fire Challenge. It is the only part of the game that (1) I definitely could not complete by simply "winging it", (2) forced me to understand and appreciate the spellcaster classes (since I brought the recommended characters) and (3) compelled me to choose my battles rather than cleaning up all possible enemies.
I was surprised by the anticlimactic final act since I was expecting an action-packed finale like that of PoE 1. As I started and completed SSS and FS right before the final act, the difficulty of the bosses there made the Guardian encounter like a stroll in the park. I'm kind of disappointed, though, that the ending was too "linear"/inevitable and was practically spoiled as early as the Ashen Maw encounter. Even the option that could theoretically alter the outcome - the one you can obtain at the end of FS - turns out to be futile.
Still, it was a great experience much like PoE 1. I hope there's a next cRPG involving Eora in the foreseeable future.