You might find these posts informative:

https://chipsandcheese.com/p/arm-or-x86-isa-doesnt-matter

https://chipsandcheese.com/p/why-x86-doesnt-need-to-die

All instructions across x86 and Arm are being decoded to micro-operations, which are implementation specific. You could have an implementation which prioritizes performance, or an implementation that prioritizes power consumption, regardless of the ISA.

Decoding instructions, particularly on a modern die, doesn’t consume a significant amount of area or power, even for complicated variable length instructions.