A constitution is not a codification of objective facts but of principles and processes for resolving disputes between people. To my mind it seems more likely that "cruel and unusual punishment" means "any punishment cruel and unusual according to community standards" than "this specific list of punishments".
No, but I also don't think a textualist interpretation of the 8th amendment proscribes only a fixed set of punishments that existed in 1789. Words like "arms" and "punishments" denote categories--kinds of things grouped by shared characteristics. Someone in 1789 would be amazed by the technology of a modern shotgun, but would perceive it shared all the characteristics of what they thought of as "arms."
Likewise, hydraulic presses did not exist in 1789, but someone from the time would have recognized putting a convict's appendages in such a machine as sharing the characteristics of what they thought of as "cruel and unusual punishment."
But the death penalty did exist in 1789, and it wasn't considered a cruel and unusual punishment. So you're not just talking about fitting new things into existing definitions, but changing the definitions themselves to include things that were previously excluded.
Its not a changing definition when the ordinary definition of a word refers to circumstances which are subject to change.