Given that you need a really simple ISA, and probably also good code density, how about a Forth CPU, like https://excamera.com/sphinx/fpga-j1.html https://excam

Given that you need a really simple ISA, and probably also good code density, how about a Forth CPU, like https://excamera.com/sphinx/fpga-j1.html https://excamera.com/sphinx/fpga-j1.html?