Alan S

Photos’ library is (largely just) a special folder structure that Finder presents as a single file and which you can explore by Ctrl-clicking and choosing ‘Show Package Contents’. So, even if Photos was discontinued at some point, like iPhoto and Aperture, you can get at the masters inside its library without needing the app.

You can work that way in Apple Photos by turning off ‘Copy items to the Photos library’ in the app’s General preferences. That’ll stop photos stored externally from being auto-uploaded to iCloud Photos but I suspect you may not care. Relevant Apple doc: https://support.apple.com/en-gb/guide/photos/pht1ed9b966d/mac

TIFF compression options were added to Photo since this thread was last touched: Choose File > Export. Select TIFF as the export format. Click More at the bottom of the Export Options panel. Set the Compression setting to ZIP, LZW or None.