Sheol in the Torah is described as the abode of both the righteous and unrighteous dead, and though the practice was forbidden the dead could be summoned to speak from there, like by the Witch King of Endor. It bears close comparison to Hades, as basically 'the grave', and so naturally an eternal abode - although the Sadducee–Pharisee ideological rift was largely about how literal the afterlife was, and how much emphasis should be on it. Gehenna refers to the Valley of Hinnom, a place cursed by a prophet because kings of Judah had sacrificed their children there, it was considered a destination of special torment, although it is not mentioned in this role in the Torah. It bears some comparison to Tartarus, the prison of the Titans below Hades.
The King James Version of the Bible translates both Sheol and Gehenna with the Anglo-Saxon word hell, and it's the only translation to do this. The original New Testament uses Gehenna when describing punishment like for breaking commandments eg with idolatry, calling it a place of destruction of the body and soul, indicating not having a place in 'the world to come', after the Resurrection. It uses Hades, in the context of the result of forgetfulness, not breaking the rules but not keeping to all the practices.
The word 'netzach' in Hebrew which is translated as eternal, is literally used to mean 'everlasting'. But, as the name of the seventh of the ten sephirot in the Kabbalah, it is used to refer to perpetuity, endurance, and to communicate the idea of long-suffering, strength, endurance unto completion or patience, and spiritually as the development of fortitude and patience to follow through on your passions. In the New Testament, it is also translated as 'patience' in the context of the 'fruits of the spirit' (Galatians 5:22).
So I would contend, if Gehenna is a place of destruction in the Bible, destruction cannot go on eternally, or it would never be completed. But in the context of contrast to the promise of eternal life, it means to go to the grave forever. It is eternal in the sense of no return, not in the sense of unlimited punishment.
In this answer I discussed the symbolic and metaphorical role of Heaven, in contrast to the Resurrection and joining the 'world to come': What are some philosophical works that explore constructing meaning in life from an agnostic or atheist view? The TLDR is that the former relates back to practice of naming constellations after heroes, so that their stories must be learned (at least by any navigator) and can become ongoing guides for life - those in the Bible who explicitly go to Heaven are exceptional saintly people, whereas the promise of eternal life meaning of Resurrection goes to those who believe (justificio sola fide). As a metaphor, we can understand that our stories can in a sense keep us alive forever (in theory), by people not only learning them but using the guidance of the stories to ask 'what would x character do here?'
I do think eternal punishment for finite crimes would be immoral. It would also seem like a waste of resources by a deity that clearly didn't want the condemned around. But, even most Christians don't get taught to distinguish Heaven and the Resurrection, and I think there is a similar confusion about going into everlasting destruction in a cursed place, rather than going to a place of eternal torment; and unfortunately Christianity just went with ignoring that. Hell could be thought of metaphorically as a place of being remembered not as a guide and inspiration, but as an example of what not to do.
The only New Testament line that seems to directly asserts Hell as eternal is:
“the smoke of their torment goes up forever and ever and they have no
rest day and night” -Revelation 14:11
And that can be understood as the consequences of bad actions emanating eternally.
To be clear, my view us not supported by any major Christian denomination as far as I know. But I don't think they read their book properly. In this answer I talk about the issues of different religions having only specific ways they can update their doctrines: The Ethics of Finding Comfort in Religion: Balancing Personal Benefits and Societal Harm I've always thought the Jewish system of rabbis having to argue everything out with references to texts, results in much more coherent theology and interpretations that also get adapted more usefully to the community, versus just having a handful of Ecumenical Councils plus infallibility of the Pope.
There are probably people now who would tell me I'm going to Hell for my views. And until a few hundred years ago I might have got fiery torment in this life, and been burned at the stake (Cathars had what look now pretty minor theological differences and got the Inquisition). But as Socrates showed with the Euthyphro Dilemma, piety must mean using our reason to understand it, not just doing what we are told.
In this answer I discuss how it's better to think about a religions in terms of how a community can be bound together and cohere around values, rather than as speculative cosmologies: Is there such a thing as weak evidence?
