i do something similar but with tags and without the merging. vim's tag navigation is very powerful. split a window with the target tag and see side by side the class and its descendant for example.

i use `venv`s for every project inside the project folder so uctags also generates tags for all libraries installed and i can "drill all the way up" to classes and definitions.

it's also possible to have the system libraries show up in the tag database, it's just a matter of telling uctags which path's, files to include/exclude or alternatively use another tag file for that, vim can use multiple tag files.

the real pain point is keeping the tag file update. gutentags makes this a bit more easy for me.