When facing a binary for which Ghidra lacks the SLEIGH specifications required to disassemble it, you hit a problem that the vast majority of reverse engineers will never encounter.
Out of an estimated 25,000 reverse engineers in the United States, only about 500 (~2%) work on this specific domain.
The Reality of Undocumented ISAs
A processor without a documented instruction set is never completely exotic. Recreating an Instruction Set Architecture (ISA) from scratch is far too expensive.
Consequently, almost all undocumented processors are derivatives of major existing families:
