For readers finding this older Mac-thumbnail question today: I have made a free extension to restore model-image Finder icons in current SketchUp versions, Mac SKP Icon Repair & Keeper 0.1.5. I am its author.
You can repair a whole library through Extensions > Repair a folder of SketchUp Finder icons… . Normal subfolders are included, without opening each model. It uses previews already embedded in the .skp files, changes icon metadata without rewriting models, and skips files without previews. It also restores the current model’s icon after ordinary saves.
The installer is in SketchUcation PluginStore as Mac SKP Icon Repair. Source, instructions and direct download are in SamTSham’s sketchup-icon-repair repository on GitHub. My announcement in Extensions & Applications Discussions links to both.
Testing covers Apple silicon with SketchUp 2021–2026; this is not a compatibility claim for the older systems discussed here. Intel hardware testing is unconfirmed. Folder repair needs Python 3 at /usr/bin/python3. It restores Finder icons, not the Quick Look service or the other interface features discussed above.