Would it be possible to upgrade to Joomla 4, leave Joomgallery 3 installed, and then upgrade to Joomgallery 4 when it is finally released? Should this preserve all images and data?
Yes, this works.
You have to uninstall all JoomGallery 3 modules and plugins but keep the component. Then do the upgrade to Joomla 4.
After upgrade, the JoomGallery views in frontend and backend will throw php errors and will not work anymore. But data is perserved.
In the meantime you could use Joomla 4 compatible image modules or plugins to show all images of a folder. Since JoomGallery is sorting the images in folders according to the category structure you can show this way gallery views with images of a specific category.