Google and Spotify have finally got around to integrating offline playback on Wear OS, now Wear. However, it seems that Garmin smartwatches will still have more Spotify functionality than their Wear OS rivals.
https://www.notebookcheck.net/Offline-Spotify-playback-is-finally-coming-to-Wear-OS-but-Garmin-may-still-have-the-edge.540132.0.html
For the offline Spotify playback feature, it is not available on some devices. But here you can use the tool DRmare Music Converter for Spotify to help you. It is a tool that can convert and download music from Spotify to plain audio formats for using on any device without any limit.
It is such good news. I have been using AudKit Spotify to MP3 Converter to download and convert Spotify songs to MP3 files before the integration of Spotify and Wear OS. Now I can directly access my Spotify library.