So I have not done anything like this, but if I were to do so, I think I would look at an Arduino. A nice convenient dev board with peripheral interfaces on it, a competent microprocessor, and plenty of ways to program it.
Specifically, whilst the majority of the Arduino programming ecosystem is based around C language, for the OP's requirements, the Arduino can definitely be programmed in assembly, which raises another important question.
Will the OP be relying on assembly language modules and libraries developed by the community? Or will the OP be coding assembly from scratch?
If the OP is looking for community assembly code examples and snippets, they may struggle to find a large codebase beyond PIC. Pretty much everyone has moved on to C.
The lack of a large assembly codebase also means less training material for AI tools, which may be more error prone with generating assembly language, compared to AI generated C code. This may be a moot point if the OP won't be using AI.