The Aramaic was Gehinnam for Greek geenna, and Sheol for Greek hades. You're right that neither of these words necessarily mean endless hellfire, and that Jesus didn't use those words to teach endlessness; but he did use other words to indicate endlessness, including by comparison to the endless life of the righteous. (I identified 45 such Scriptures across all related concepts.) At the same time, this concept has indeed been abused for power dynamics for many centuries.

Mark 9:47-48 quotes Is. 66 and emphasizes the unchanging nature of the worm and fire; comparison to other uses of "unquenchable" indicates a fire that does not go out. Since this is compared to entering the kingdom which refers to the next life, it is read to be more than a metaphor. Now this is the only one I'm confident of that uses geenna with a term of endlessness, but if you want to see the whole list here it is. That uses ordinary grammaticohistorical assumptions about the text, but I haven't found a way to explain away that many references as metaphorical, temporary, or otherwise deprecated.

The Catholics and some others are adopting a doctrine of hopeful universalism that intends to be consistent with the texts, and having studied it I can't gainsay it as long as nobody gets dogmatic about the distant future. But I'm just letting you know the question is much more complicated than dismissing the incorrect veneer that you rightly move past. The fact that we've combined concept in English that were separate in Aramaic and Greek doesn't mean that this misstep annuls what the originals say. Suffice that Moses teaches there is fire in sheol and this was enough to scare the covenant Jews for 1500 years before it got used to scare Gentiles.