Easiest would be to invent my own instruction set
uh, ok, we might come from very different experiences here…
With easiest I mean the least amount of instructions.
That's not necessarily the easiest to implement. Often, having more instructions is a good complexity tradeoff compared to having more complex instructions.
So my question is, what is the easiest instruction set/architecture that has a (hopefully stable) C++ and/or C compiler?
This sounds like no job for C++, so let's concentrate on C. (If you don't understand the difference having C++ RAII paradigm makes, you might not be in the optimum position to design your own ISA.)
Puh, some microcontroller instruction set that is early, but not too early (because too early would imply "designed around the limitations of digital logic of that time, like e.g. 8051).
AVR might be a good choice, though I personally don't like that too much.
I hear Zilog Z80 is easy to implement (there's really several Z80 implementations out there), but it's pretty ancient, and not very comfortable (being from the mid-70s).
If you really just want a small core to control what your system is doing, why not pick one of the many processor core designs that are out there?
For example, RISC-V is a (fairly complex) instruction set architecture, with mature compilers, and many open source implementations. For a minimal FPGA core, picoRV32 would probably the core of choice. And on a computer, you'd just run QEMU.