There is at least one more layer, which lies behind the abstraction boundary the ISA presents. E.g. x86-64 specifies only 16 general purpose registers, but the Zen 4 has 224. Even as far back as Pentium II the CPU had more registers than the ISA.
You’re not even close to hitting rock bottom on explanations of the layers below until you find yourself comparing proposed theories of quantum gravity :-)
I don't think ignoring negligible effects means you're a significant distance from the bottom. You can semi-skim through quantum effects and still have enough detail to stretch your scale to the bottom.
