Writing backends to integration into compiler stacks like LLVM is as challenge unless one is quite deep into compilers.
If the idea is just to learn how they work, it is easier to just dump an IR that is compatible with a macro assembler, then just call the assembler with your macro definitions on the generated IR.
It won't win prices in performance, but one gets to learn how things work and will have a compiler with proper binaries at the end, and one can fine tune the macros to improve the quality of the code anyway.
If the idea is just to learn how they work, it is easier to just dump an IR that is compatible with a macro assembler, then just call the assembler with your macro definitions on the generated IR.
It won't win prices in performance, but one gets to learn how things work and will have a compiler with proper binaries at the end, and one can fine tune the macros to improve the quality of the code anyway.