Generally speaking, I think this project is suitable for beginners, as you’ll naturally learn about all the important parts of LLVM along the way. Having said that, it seems like there is a lot of interest in this project, and having some prior contribution to LLVM is often a deciding factor in such cases.
Next to the resources you linked, the only relevant documentation I can think of is The LLVM Target-Independent Code Generator — LLVM 19.0.0git documentation, in particular the “Instruction Selection” section talking about SelectionDAG.
To get started working on LLVM in general, taking a look at good first issues is probably worth a shot.