OK looks like I've sort of got everything back. No artwork :( But the playlists are there. Here's what I did:
See What happened to iTunes? - Apple Support for a brief overview of changes that took place with the release of macOS 10.15 Catalina. If you are moving your iTunes library from an older version of macOS to Catalina or later and want to retain ratings, play counts, playlists, etc. that are not stored in tags then you can hold down Alt/Option as you launch the Music and TV apps for the first time. You can use the Choose Library function to select your old iTunes Library.itl file and convert to the new library format for each app. This only needs to be done once for Music, and again for TV if you have video content. The conversion will leave everything stored in the old iTunes Media or iTunes Music folder. You may want to rationalise your media library so that it would look the same as if it had always been built using the newer apps, and you can remove or archive any legacy content not managed by either app. This should also make migration easier in future. These library conversions should happen automatically if you upgrade a computer running Mojave or earlier to Catalina or later, but can be performed manually if required.