>>328186787
https://github.com/dabeschte/VRArmIK and its associated research paper https://www.markussteinberger.net/papers/HumanUpperBodyIK.pdf might help, they have some interesting heuristics for shoulders and elbows. The unity implementation isn't very good imo but in theory you could redo it yourself. https://github.com/exokitxr/avatars has an implementation of it in JS, also at https://avatars.exokit.org

If you search for FABRIK, you can also find more resources on its pretty simple approach to IK calculation. I don't know how useable it is genetically on humanoid rigs though.

Wish I still had time to work on this stuff myself.