There are many times when I can't finish all of the changes that I'd like to make in one sitting, and so it would be convenient to be able to work offline and upload changes all at once. Is this possible?

Changes are made directly to the site. However, there are a few approaches which you may find helpful:

If you are building out a site or doing a major overhaul, and you don't want visitors seeing what you're doing until it's finished, you can put your site into maintenance mode.

If you are adding content that you are not ready to show the world, uncheck the "published" box under publishing settings. You can also use an unpublished page as a temporary place to make edits. Copy your content from the live page to the unpublished page, make your changes to the unpublished page, and copy the finished content back to the live page when you're ready. You can use the "create new revision" option if you'd like to be able to "undo" changes to a page.

If you are working on block content that you don't want others to see, you can set the block's visibility to show only for the content manager role.