Hi James and Friends
I did this exercise, before loading the New Extensions
and in particular (extractPt ...) ;
but the result is similar to your ...
The real problem is that you can't get it to flow together
multiinput ... (Grasshopper with its branching, it could be
an inspiration to be able to reach this milestone in the future)

the file at this link:
http://www.mediafire.com/file/8d3b6uwm1a6455y/Baricenter_and_BBox_Center.zip/file
Have a nice Week-end to all
alberto