You can try to reduce number of images. WordPress creates lots of different image sizes (up to several of every image you uploaded), but actually you do not use all of them in most cases. Also during years you probably deleted/edited some posts/images, but some images are probably still on your disk, even not used in posts now.
So you can delete images not used anymore, or delete some resized images which you do not use. This will give you time to figure out how to move your break your images into more sub folders, and also you can delete few thousands of images so you will not need to do it. So, to find which images you can delete you can do it with plugin or manually:
Deleting images using plugin:
You can use this plugin, it will search your database and look if image is inserted into any post (in content, as featured image, in any custom field, anywhere…) or as background…
If image is not used anywhere it will give you option to delete it. You will get list of all images on your site not used anymore, so you can safely delete them.
https://wordpress.org/plugins/dnui-delete-not-used-image-wordpress/
and this, still work, even it is outdated
https://wordpress.org/plugins/wordpress-uploaded-files-cleaner/
Deleting images manually:
You can also search through your media library for images which are unattached to posts and pages.
Go to media library and click the “unattached” it will display all images that may be displayed on other parts of your site or not used.