The Admiral class as a global commerce-war weapon

Were the Admiral-class battlecruisers designed mainly as an escalation against American ships?

No. They were conceived for global mobility and commerce warfare against the next plausible naval powers after Germany.

The Admiral class and later G3 thinking responded to oceanic manoeuvre, commerce protection, and commerce raiding rather than a renewed North Sea battle line. Their heavy guns forced an enemy to send a battleship after them, while their speed let them destroy convoys and threaten maritime trade across a global operating area.