Now I’m contemplating just how small and light of an instrument could be carried on a Starlink-style satellite that could detect a large ship. A smallish COTS telescope, e.g. a Celestron 8SE ($1700 retail) could easily see a ship from the Starlink constellation altitude.

Never mind that the Starlink radio arrays are, well, radio arrays that quite effectively cover the whole planet. If you think of each satellite as a radio telescope, its resolution is crap and probably cannot disambiguate a carrier group from anything else (at least according to disclosed specs). But it would be quite interesting to build a synthetic aperture array out of multiple satellites. This would rely on emissions from the ships themselves, but I bet it could be done and could locate ships quite nicely.