plugin for synfig.
similary like shape key in Blender or Smartbone in MOHO
need Gtk3 PyGObject, IK angle Converter.
download $1 plugin Smartkey
https://mailcatrobert.gumroad.com/l/mghttf
synfig 1.5.4 unofficial try and Ik angle:
demo: https://youtu.be/kgnKCwXtjIQ
https://mailcatrobert.gumroad.com/l/votsfv
appimage download: https://mailcatrobert.gumroad.com/l/mdmti