Can metadata be expanded to include more ISTP fields? And can Opus read metadata from PSB files?

If File Explorer has columns for them, you can add them to Opus via Preferences / File Display Columns / Shell Properties.

For other data that Opus doesn't display natively, using ExifTool to populate columns for the data is probably your best bet: ExifTool Custom Columns

Both will add columns you can add to the file display or include in info-tips and tiles. They won't affect the Metadata Panel, which only shows metadata Opus knows how to edit (plus a few read-only fields to provide context).