I wrote my first compiler in M68000 assembler in a similar fashion to your steps. I started by allowing it to recognize M68000 assembler opcodes, and when found it'd just copy the line to output, otherwise it'd parse and compile the line. You could use register names and sizes directly in the statements.

So e.g.

    D0.W = 5

Would translate into:

    MOVE.W #5, D0

I didn't use macros though - handling basic argument passing is simple enough.

As soon as I had the basics of procedures/functions, simple expressions and argument passing in place I started rewriting the compiler using it.