Is it possible to Filter System Clipboard with Lowercase First except when the first character is "I" (a capital "i"), in which case the capitalisation should be retained?
I don't think you can customize the filters but this should get your started.
Lowercase First Macro (v11.0.2)
Lowercase First.kmmacros (2.0 KB)
To use this version, copy a selection, run the macro and paste the clipboard over the selection.