No, you didn’t understand correctly, Joanna. I didn’t say anything about adjustments, which live very happily in .dop files. I said all metadata belongs in .xmp files. I also accounted for speed in searches and other data reading:
The database can store metadata for quick display but at any point the photographer decides to edit the metadata, the metadata must be refreshed from the XMP.
When a photographer decides to edit the metadata of an image, the milliseconds it takes to read an .xmp file from an SSD are of no consequence. There would be no need to reread .xmp files just to perform a search. A photographer should be able to force a reread of the .xmp files. Or PhotoLab could do so intelligently by keeping timestamps on modifications of the .xmp files in the database. When PhotoLab detects that the .xmp files have been modified since the data was stored in the database that could trigger a background read of the .xmp files in that folder.
There are ways to keep metadata up to date and in sync. Simplifying sync means avoiding it by making a single version canonical. The canonical version of metadata should always be the .xmp sidecar as that’s where the other well-behaved photo applications which manage metadata store metadata.
@StevenL I’ve just solved most of your issues with metadata sync and inconsistency in the metadata between applications. I’ll accept a working version of PhotoLab 5 on Mojave as payment. You’re welcome.