Thank you for clarifying some things... and I hope you don't find I'm "pushing" too much :-)

As far I can tell, using "-all:all" behaves as expected and I always specify what to read/write when using ExifTool (despite missing two DateTimes, which will be corrected in v7.15, you said).
It is just how EXIF DateTimes are written (using -tagsfromfile) when tag/group isn't specified: EXIF:ModifyDate has TimeZone, while EXIF:DateTimeOriginal and EXIF:CreateDate haven't. But, speaking for me (as I always specify Group/TagName), it really doesn't bother me.

I realy don't know if there's something "needed" to be changed. Despite above "issue", ExifTool (at least how I use it) always does as expected:
1. EXIF DateTimes written or copied from somewhere, never have TimeZone trailers,
2. XMP DateTimes written or copied from somewhere, always have TimeZone (or "Z" if there's none)
IMO, that's how it should be. And I see "-n" option as a feature when writing DateTimes -to override default DateTime format.
So, as long ExifTool does as said above (1. and 2.), I wouldn't change nothing in this regard.

Thank you for patience,
Bogdan