If a person’s life is not their own to take then they have no autonomy at all
That’s just not right. Autonomy isn’t some absolute, all or nothing thing. If it was, then everybody would have “no autonomy at all”, because we’re not allowed to commit crimes.
Of the full range of possible actions, killing yourself is a relatively small portion of those. Considering that death eliminates all possible future actions, I’d argue that preventing a suicide (of a person that’s not dying anyway) actually preserves more autonomy than the alternative.