Another approach is to do it with scripts.
Schematic symbols in KiCad are in a pretty simple text format. Some years ago I wanted to experiment a bit with python, and in a single afternoon I wrote a simple python script to generate a library with some 40 connectors.
With such a script you can of course add all the custom fields you want.
You can find the script in:
Some words of caution:
KiCad V6 is now not far away (maybe half a year) and it has significant changes in the file formats especially for schematics, so now is not such a good time to invest time in learning those formats.
Current schematic symbol libraries are a bit weird. You have the symbol itself in a .lib file, and some additional file with auxillary information, I think it’s an .scm file. But I don’t really know what’s in it.