I deleted about 400 MB of files using the "Storage" page in the left sidebar of my Google Drive ( https://drive.google.com/drive/u/0/quota ) and now I can edit my Sheet again. My "storage" page in settings (https://drive.google.com/settings/storage) still shows 18.86 GB of 28 GB used.

Honestly, I regret ever signing up for the product that is now "legacy G Suite". It's been a rough experience over the years and there doesn't seem to be an easy way to migrate from this product to a regular Google account. (I looked into this back when they were threatening to discontinue the product.)