I don't think you can criticize "deus ex machina" as a plot device when it's, from go, literally "ex deus" - Gandalf is on a mission (/ missions) from god, and Eru's not gonna let him get off so easily.

It's arguable whether you want to describe the wizards as 'Mortal' in the first place.

The return of Gandalf never much bothered me, it fit well enough.

Just because it's literally ex deus doesn't make it less ex deus.

Ex deus is cheap

The more critical part is "from go" - deus ex machina is considered bad because it's a narrative rupture. In a story of wars openly involving divine beings, there's no such discontinuity. It is also usually associated with stories that wrap up a bit too tidily, and that's also not the case in LotR.

Gandalf dies 2/6 of the way through the story and comes back 3/6 of the way through. His death may be the climax of the first two books, but his return is merely a plot beat and many of the remaining problems have yet to even be revealed, let alone solved.

You don't think the plot was put out in advance for Gandalf to not die in Moria? Or what do you mean?

I guess he had to disappear for the fellowship to break up.

If I remember correctly, you don't get to know that he technically did die in Moria. That is background lore.

Tricking the reader to think he died to let him return more powerful when he is needed.

That's comic book logic.

Boromir's death was far better plotwise.

He doesn't return more powerful, he returns with a different mandate. He is no more or less powerful as he was before, rather he is "licensed" by Eru to reveal more of it as Saruman has abdicated that role, and Gandalf had been the only faithful Istari of the five - dying being a critical part of that.

According to Tolkien he is more powerful

>What Gandalf did was what the Authority wished. The "wizards" were failures as they had been and needed enhancement to match the gravity of the crisis. Gandalf's sacrifice was accepted so he was returned in an enhanced state (more power and wisdom). The old Gandalf could not have dealt with Théoden or Saruman as he was able after his enhancement

https://tolkiengateway.net/wiki/Letter_156

It was based on the Bible and the Resurrection. Tolkien was an observant Catholic and openly talked about the religious elements in LOTR. For example, the description of Gandalf's white rainments are similar to Jesus's in the Gospels.