The reason computer scientists aren’t interested in defining a minimum instruction set computer is because it’s a solved problem: see “One-instruction set computer”. There are a variety of single instructions which are Turing-complete, such as:

- Subtract and branch if less than or equal to zero
- Subtract and branch if negative
- Subtract if positive else branch
- Reverse subtract and skip if borrow

One advantage of a single instruction set computer is that you don’t need an op-code in the instruction, since there’s only one.