The practical advice is: don’t manually manage individual metadata fields if you can avoid it. Use a DAM/metadata-capable app such as digiKam, darktable, Lightroom, or Bridge and let it write the appropriate EXIF/IPTC/XMP fields for you.

Trying to keep multiple description fields synchronized yourself is error-prone, and software support varies. In general, XMP is the safest modern choice if you must pick one manually; EXIF and especially legacy IPTC/IIM are less ideal for rich descriptive text. Reported IPTC length limits are often ignored by modern software, but relying on old legacy fields for long descriptions is still not the best long-term strategy.

If broad usability matters, also consider simple folder and file naming for basic context, and optionally keep a separate index/spreadsheet for more detailed family-history notes. External records are often easier to search and manage across many images, while embedded metadata is best for concise captions and core info that should travel with the file.

So: use a DAM, prefer XMP-based metadata, don’t obsess over syncing every legacy field manually, and keep extensive notes externally if needed.